castle/BEANCOUNT_PATTERNS.md

# Beancount Patterns Analysis for Castle Extension

## Overview

After analyzing the [Beancount repository](https://github.com/beancount/beancount), I've identified several excellent design patterns and architectural decisions that we should adopt or consider for the Castle Accounting extension.

## Key Patterns to Adopt

### 1. **Immutable Data Structures with NamedTuple**

**Beancount Pattern:**
```python
class Transaction(NamedTuple):
    """A transaction with immutable attributes"""
    meta: Meta
    date: datetime.date
    flag: Flag
    payee: Optional[str]
    narration: str
    tags: frozenset[str]
    links: frozenset[str]
    postings: list[Posting]

class Posting(NamedTuple):
    """Individual posting within a transaction"""
    account: Account
    units: Optional[Amount]
    cost: Optional[Union[Cost, CostSpec]]
    price: Optional[Amount]
    flag: Optional[Flag]
    meta: Optional[Meta]
```

**Benefits:**
- Immutability prevents accidental modifications
- NamedTuple provides free `__repr__`, `__eq__`, `__hash__`
- Type hints improve IDE support and catch errors
- More memory efficient than regular classes
- Thread-safe by design

**Castle Application:**
```python
# In models.py
from typing import NamedTuple, Optional
from datetime import datetime
from decimal import Decimal

class ImmutableJournalEntry(NamedTuple):
    """Immutable representation of a journal entry"""
    id: str
    description: str
    entry_date: datetime
    created_by: str
    created_at: datetime
    reference: Optional[str]
    lines: tuple[ImmutableEntryLine, ...]  # Tuple, not list!
    meta: dict[str, Any]  # Could store filename, lineno for audit trail

class ImmutableEntryLine(NamedTuple):
    """Immutable representation of an entry line"""
    id: str
    journal_entry_id: str
    account_id: str
    debit: int
    credit: int
    description: Optional[str]
    metadata: dict[str, Any]
    flag: Optional[str]  # Like Beancount: '!', '*', etc.

# Conversion functions
def entry_to_immutable(entry: JournalEntry) -> ImmutableJournalEntry:
    """Convert mutable Pydantic model to immutable NamedTuple"""
    return ImmutableJournalEntry(
        id=entry.id,
        description=entry.description,
        entry_date=entry.entry_date,
        created_by=entry.created_by,
        created_at=entry.created_at,
        reference=entry.reference,
        lines=tuple(line_to_immutable(line) for line in entry.lines),
        meta={}
    )
```

**Recommendation:** Use immutable NamedTuples for internal processing, Pydantic models for API I/O.

---

### 2. **Plugin Architecture**

**Beancount Pattern:**
```python
# In plugins/check_commodity.py
__plugins__ = ('validate_commodity_directives',)

def validate_commodity_directives(entries, options_map, config):
    """
    Plugin that validates all commodities have declarations.

    Args:
        entries: List of directive entries
        options_map: Parser options
        config: Plugin-specific configuration

    Returns:
        (entries, errors) tuple
    """
    errors = []
    # ... validation logic ...
    return entries, errors
```

**Castle Application:**
```python
# Create plugins/ directory
# lnbits/extensions/castle/plugins/__init__.py

from typing import Protocol, Tuple, List, Any

class CastlePlugin(Protocol):
    """Protocol for Castle plugins"""

    def __call__(
        self,
        entries: List[JournalEntry],
        settings: dict[str, Any],
        config: dict[str, Any]
    ) -> Tuple[List[JournalEntry], List[dict]]:
        """
        Process entries and return modified entries + errors.

        Args:
            entries: Journal entries to process
            settings: Castle settings
            config: Plugin-specific configuration

        Returns:
            (modified_entries, errors) tuple
        """
        ...

# Example plugins:

# plugins/check_balanced.py
__plugins__ = ('check_all_balanced',)

def check_all_balanced(entries, settings, config):
    """Verify all journal entries have debits = credits"""
    errors = []
    for entry in entries:
        total_debits = sum(line.debit for line in entry.lines)
        total_credits = sum(line.credit for line in entry.lines)
        if total_debits != total_credits:
            errors.append({
                'entry_id': entry.id,
                'message': f'Unbalanced entry: debits={total_debits}, credits={total_credits}',
                'severity': 'error'
            })
    return entries, errors

# plugins/auto_tags.py
__plugins__ = ('auto_add_tags',)

def auto_add_tags(entries, settings, config):
    """Automatically add tags based on rules"""
    tag_rules = config.get('rules', {})
    for entry in entries:
        for pattern, tag in tag_rules.items():
            if pattern in entry.description.lower():
                if 'tags' not in entry.meta:
                    entry.meta['tags'] = set()
                entry.meta['tags'].add(tag)
    return entries, []

# plugins/check_receivables.py
def check_receivable_limits(entries, settings, config):
    """Warn if receivables exceed configured limits"""
    errors = []
    max_per_user = config.get('max_receivable_per_user', 1_000_000)

    # Calculate current receivables per user
    receivables = {}
    for entry in entries:
        for line in entry.lines:
            if 'Accounts Receivable' in line.account_name:
                user_id = extract_user_from_account(line.account_name)
                receivables[user_id] = receivables.get(user_id, 0) + line.debit - line.credit

    for user_id, amount in receivables.items():
        if amount > max_per_user:
            errors.append({
                'user_id': user_id,
                'message': f'Receivable {amount} sats exceeds limit {max_per_user}',
                'severity': 'warning'
            })

    return entries, errors
```

**Plugin Manager:**
```python
# plugins/manager.py
import importlib
from pathlib import Path

class PluginManager:
    def __init__(self, plugin_dir: Path):
        self.plugins = []
        self.load_plugins(plugin_dir)

    def load_plugins(self, plugin_dir: Path):
        """Discover and load all plugins"""
        for plugin_file in plugin_dir.glob('*.py'):
            if plugin_file.name.startswith('_'):
                continue

            module_name = f"castle.plugins.{plugin_file.stem}"
            module = importlib.import_module(module_name)

            if hasattr(module, '__plugins__'):
                for plugin_name in module.__plugins__:
                    plugin_func = getattr(module, plugin_name)
                    self.plugins.append((plugin_name, plugin_func))

    def run_plugins(
        self,
        entries: List[JournalEntry],
        settings: dict,
        plugin_configs: dict[str, dict]
    ) -> Tuple[List[JournalEntry], List[dict]]:
        """Run all plugins in sequence"""
        all_errors = []

        for plugin_name, plugin_func in self.plugins:
            config = plugin_configs.get(plugin_name, {})
            entries, errors = plugin_func(entries, settings, config)
            all_errors.extend(errors)

        return entries, all_errors
```

**Benefits:**
- Extensibility without modifying core code
- Easy to enable/disable validation rules
- Community can contribute plugins
- Each plugin is independently testable

---

### 3. **Inventory System for Position Tracking**

**Beancount Pattern:**
```python
class Inventory(dict[tuple[str, Optional[Cost]], Position]):
    """
    Tracks positions keyed by (currency, cost).
    Handles lot matching with FIFO, LIFO, HIFO strategies.
    """

    def add_position(self, position: Position):
        """Add or merge a position"""
        key = (position.units.currency, position.cost)
        if key in self:
            self[key] = self[key].add(position)
        else:
            self[key] = position

    def reduce(self, reducer_func):
        """Convert inventory using a reduction function"""
        return sum((reducer_func(pos) for pos in self), Inventory())

    def get_currency_units(self, currency: str) -> Decimal:
        """Get total units of a specific currency"""
        return sum(
            pos.units.number
            for (curr, cost), pos in self.items()
            if curr == currency
        )
```

**Castle Application:**
```python
# core/inventory.py
from decimal import Decimal
from typing import Optional, Dict, Tuple
from dataclasses import dataclass

@dataclass(frozen=True)
class CastlePosition:
    """A position in the Castle inventory"""
    currency: str  # "SATS", "EUR", "USD"
    amount: Decimal
    cost_currency: Optional[str] = None  # Original currency if converted
    cost_amount: Optional[Decimal] = None  # Original amount
    date: Optional[datetime] = None
    metadata: Dict[str, Any] = None

class CastleInventory:
    """
    Track user balances across multiple currencies with conversion tracking.
    Similar to Beancount's Inventory but optimized for Castle's use case.
    """

    def __init__(self):
        self.positions: Dict[Tuple[str, Optional[str]], CastlePosition] = {}

    def add_position(self, position: CastlePosition):
        """Add or merge a position"""
        key = (position.currency, position.cost_currency)

        if key in self.positions:
            existing = self.positions[key]
            self.positions[key] = CastlePosition(
                currency=position.currency,
                amount=existing.amount + position.amount,
                cost_currency=position.cost_currency,
                cost_amount=(
                    (existing.cost_amount or Decimal(0)) +
                    (position.cost_amount or Decimal(0))
                ),
                date=position.date,
                metadata={**(existing.metadata or {}), **(position.metadata or {})}
            )
        else:
            self.positions[key] = position

    def get_balance_sats(self) -> Decimal:
        """Get total balance in satoshis"""
        return sum(
            pos.amount
            for (curr, _), pos in self.positions.items()
            if curr == "SATS"
        )

    def get_balance_fiat(self, currency: str) -> Decimal:
        """Get balance in specific fiat currency from cost metadata"""
        return sum(
            pos.cost_amount or Decimal(0)
            for (curr, cost_curr), pos in self.positions.items()
            if cost_curr == currency
        )

    def to_dict(self) -> dict:
        """Export to dictionary"""
        return {
            "sats": float(self.get_balance_sats()),
            "fiat": {
                curr: float(self.get_balance_fiat(curr))
                for curr in set(
                    cost_curr
                    for _, cost_curr in self.positions.keys()
                    if cost_curr
                )
            }
        }

# Usage in balance calculation:
async def get_user_inventory(user_id: str) -> CastleInventory:
    """Calculate user's inventory from journal entries"""
    inventory = CastleInventory()

    user_accounts = await get_user_accounts(user_id)
    for account in user_accounts:
        entry_lines = await get_entry_lines_for_account(account.id)

        for line in entry_lines:
            # Add as position
            metadata = json.loads(line.metadata) if line.metadata else {}

            if line.debit > 0:
                inventory.add_position(CastlePosition(
                    currency="SATS",
                    amount=Decimal(line.debit),
                    cost_currency=metadata.get("fiat_currency"),
                    cost_amount=Decimal(metadata.get("fiat_amount", 0)),
                    date=line.created_at,
                    metadata=metadata
                ))

            if line.credit > 0:
                inventory.add_position(CastlePosition(
                    currency="SATS",
                    amount=-Decimal(line.credit),
                    cost_currency=metadata.get("fiat_currency"),
                    cost_amount=-Decimal(metadata.get("fiat_amount", 0)),
                    date=line.created_at,
                    metadata=metadata
                ))

    return inventory
```

**Benefits:**
- Cleaner abstraction for balance calculations
- Separates position tracking from database queries
- Easier to add lot tracking if needed later
- Consistent with accounting standards

---

### 4. **Meta Attribute Pattern**

**Beancount Pattern:**
Every directive has a `meta: dict[str, Any]` attribute that stores:
- `filename`: Source file
- `lineno`: Line number
- Custom metadata like tags, links, notes

**Castle Application:**
```python
class JournalEntryMeta(BaseModel):
    """Metadata for journal entries"""
    filename: Optional[str] = None  # For imported entries
    lineno: Optional[int] = None
    source: str = "api"  # "api", "import", "plugin", "migration"
    created_via: str = "web_ui"  # "web_ui", "cli", "script"
    tags: set[str] = set()  # e.g., {"recurring", "automated"}
    links: set[str] = set()  # Link to other entries
    reviewed: bool = False
    reviewed_by: Optional[str] = None
    reviewed_at: Optional[datetime] = None
    notes: Optional[str] = None

# Add to journal_entries table:
"""
ALTER TABLE journal_entries ADD COLUMN meta TEXT DEFAULT '{}';
"""

# Usage:
entry = await create_journal_entry(
    data=entry_data,
    created_by=user_id,
    meta={
        "source": "api",
        "created_via": "web_ui",
        "tags": ["manual", "receivable"],
        "notes": "User requested invoice #1234"
    }
)
```

**Benefits:**
- Audit trail for compliance
- Better debugging ("where did this entry come from?")
- Support for tags and categorization
- Can track review/approval workflow

---

### 5. **Balance Assertion Pattern**

**Beancount Pattern:**
```beancount
2025-10-22 balance Assets:Lightning:Balance  268548 SATS
```

This asserts that the account balance should be exactly 268,548 sats on that date. If it's not, Beancount throws an error.

**Castle Application:**
```python
# models.py
class BalanceAssertion(BaseModel):
    """Assert expected balance at a specific date"""
    id: str
    date: datetime
    account_id: str
    expected_balance: int  # in satoshis
    tolerance: int = 0  # Allow +/- this much difference
    checked_balance: Optional[int] = None
    difference: Optional[int] = None
    status: str = "pending"  # pending, passed, failed
    created_by: str
    created_at: datetime

# API endpoint
@castle_api_router.post("/api/v1/assertions/balance")
async def create_balance_assertion(
    data: CreateBalanceAssertion,
    wallet: WalletTypeInfo = Depends(require_admin_key),
) -> BalanceAssertion:
    """Create a balance assertion for reconciliation"""
    assertion = BalanceAssertion(
        id=urlsafe_short_hash(),
        date=data.date,
        account_id=data.account_id,
        expected_balance=data.expected_balance,
        tolerance=data.tolerance,
        created_by=wallet.wallet.user,
        created_at=datetime.now()
    )

    # Check balance immediately
    actual_balance = await get_account_balance_at_date(
        data.account_id,
        data.date
    )

    difference = abs(actual_balance - data.expected_balance)

    assertion.checked_balance = actual_balance
    assertion.difference = actual_balance - data.expected_balance
    assertion.status = "passed" if difference <= data.tolerance else "failed"

    await db.insert("balance_assertions", assertion)

    if assertion.status == "failed":
        raise HTTPException(
            status_code=HTTPStatus.CONFLICT,
            detail=f"Balance assertion failed: expected {data.expected_balance}, "
                   f"got {actual_balance}, difference {assertion.difference}"
        )

    return assertion

# Periodic check job
async def check_all_balance_assertions():
    """Recheck all balance assertions (run daily)"""
    assertions = await db.fetchall(
        "SELECT * FROM balance_assertions WHERE status = 'pending'"
    )

    for assertion in assertions:
        actual_balance = await get_account_balance_at_date(
            assertion.account_id,
            assertion.date
        )

        difference = abs(actual_balance - assertion.expected_balance)
        status = "passed" if difference <= assertion.tolerance else "failed"

        await db.execute(
            """
            UPDATE balance_assertions
            SET checked_balance = :actual, difference = :diff, status = :status
            WHERE id = :id
            """,
            {
                "actual": actual_balance,
                "diff": actual_balance - assertion.expected_balance,
                "status": status,
                "id": assertion.id
            }
        )
```

**Benefits:**
- Catch data entry errors early
- Reconciliation checkpoints
- Build confidence in accounting accuracy
- Required for external audits

---

### 6. **Hierarchical Account Names**

**Beancount Pattern:**
```
Assets:US:BofA:Checking
Assets:US:ETrade:Cash
Assets:US:ETrade:ITOT
Liabilities:US:CreditCard:Amex
```

Accounts are organized hierarchically with `:` separator.

**Castle Application:**
```python
# Currently: "Accounts Receivable - af983632"
# Better:    "Assets:Receivable:User-af983632"

# Update account naming:
ACCOUNT_HIERARCHY = {
    "asset": "Assets",
    "liability": "Liabilities",
    "equity": "Equity",
    "revenue": "Income",  # Beancount uses "Income" not "Revenue"
    "expense": "Expenses"
}

def format_account_name(
    account_type: AccountType,
    base_name: str,
    user_id: Optional[str] = None
) -> str:
    """Format account name in hierarchical style"""
    root = ACCOUNT_HIERARCHY[account_type.value]

    # Convert "Accounts Receivable" -> "Receivable"
    # Convert "Food & Supplies" -> "Food:Supplies"
    base_clean = base_name.replace("Accounts ", "").replace(" & ", ":")

    if user_id:
        return f"{root}:{base_clean}:User-{user_id[:8]}"
    else:
        return f"{root}:{base_clean}"

# Examples:
# "Assets:Receivable:User-af983632"
# "Liabilities:Payable:User-af983632"
# "Equity:MemberEquity:User-af983632"
# "Income:Accommodation"
# "Expenses:Food:Supplies"
# "Assets:Lightning:Balance"
```

**Benefits:**
- Standard accounting hierarchy
- Better organization for reporting
- Easier to generate balance sheet (all Assets:*, all Liabilities:*, etc.)
- Compatible with Beancount import

---

### 7. **Flags for Transaction Status**

**Beancount Pattern:**
```beancount
2025-10-22 * "Confirmed transaction"
  Assets:Checking  -100 USD
  Expenses:Food     100 USD

2025-10-23 ! "Pending/uncleared transaction"
  Assets:Checking  -50 USD
  Expenses:Gas      50 USD
```

Flags: `*` = cleared, `!` = pending, `#` = flagged for review

**Castle Application:**
```python
# Add flag field to journal_entries
class JournalEntryFlag(str, Enum):
    CLEARED = "*"      # Fully reconciled
    PENDING = "!"      # Not yet confirmed
    FLAGGED = "#"      # Needs review
    VOID = "x"         # Voided entry

class JournalEntry(BaseModel):
    # ... existing fields ...
    flag: JournalEntryFlag = JournalEntryFlag.CLEARED

# Usage:
# Lightning payment: flag="*" (immediately cleared)
# Manual payment request: flag="!" (pending approval)
# Suspicious entry: flag="#" (flagged for review)
# Reversed entry: flag="x" (voided)

# UI: Show icon based on flag
# * = green checkmark
# ! = yellow warning
# # = red flag
# x = gray strikethrough
```

**Benefits:**
- Visual indication of entry status
- Filter by flag in queries
- Standard accounting practice
- Supports reconciliation workflow

---

### 8. **Decimal for Money, Not Float**

**Beancount Pattern:**
```python
from decimal import Decimal

# All amounts use Decimal, never float
amount = Decimal("19.99")
```

**Castle Current Issue:**
We're using `int` for satoshis (good!) but `float` for fiat amounts (bad!).

**Fix:**
```python
from decimal import Decimal

class ExpenseEntry(BaseModel):
    amount: Decimal  # Not float!
    # ...

class ReceivableEntry(BaseModel):
    amount: Decimal  # Not float!
    # ...

# Metadata storage
metadata = {
    "fiat_amount": str(Decimal("250.00")),  # Store as string to preserve precision
    "fiat_rate": str(Decimal("1074.192")),
}

# When reading:
fiat_amount = Decimal(metadata["fiat_amount"])
```

**Benefits:**
- Prevents floating point rounding errors
- Required for financial applications
- Exact decimal arithmetic
- `Decimal("0.1") + Decimal("0.2") == Decimal("0.3")` (float doesn't!)

---

### 9. **Query Language (Future Enhancement)**

**Beancount Pattern:**
```sql
-- Beancount Query Language (BQL)
SELECT account, sum(position) AS balance
WHERE account ~ 'Assets:'
GROUP BY account
ORDER BY balance DESC;

SELECT date, narration, position
WHERE account = 'Assets:Checking'
  AND date >= 2025-01-01;
```

**Castle Application (Future):**
```python
# Add query endpoint
@castle_api_router.post("/api/v1/query")
async def execute_query(
    query: str,
    wallet: WalletTypeInfo = Depends(require_invoice_key),
) -> dict:
    """
    Execute a query against accounting data.
    Supports SQL-like syntax for reporting.
    """
    # Parse query (could use SQLAlchemy for this)
    # Apply user permissions (filter to user's accounts)
    # Execute and return results
    pass

# Example queries users could run:
# - "Show all expenses in October"
# - "What's my average monthly spending on food?"
# - "Show all receivables older than 30 days"
```

**Benefits:**
- Power users can build custom reports
- Reduces need for custom endpoints
- Flexible analytics

---

## Structural Improvements

### 1. **Separate Core from API**

**Beancount Structure:**
```
beancount/
  core/           # Pure data structures and logic
    data.py       # Immutable data types
    inventory.py  # Position tracking
    account.py    # Account operations
  parser/         # Text file parsing
  plugins/        # Validation and transformation
  scripts/        # CLI tools
  tools/          # Reporting and analysis
```

**Castle Should Adopt:**
```
castle/
  core/              # NEW: Pure accounting logic
    __init__.py
    inventory.py     # CastleInventory for position tracking
    balance.py       # Balance calculation logic
    validation.py    # Entry validation (debits=credits, etc)
    account.py       # Account hierarchy and naming
  models.py          # Pydantic models (API I/O)
  crud.py            # Database operations
  views_api.py       # API endpoints
  plugins/           # NEW: Validation plugins
    __init__.py
    check_balanced.py
    check_receivables.py
  services.py        # Business logic
  migrations.py
  templates/
  static/
```

**Benefits:**
- Core logic is independent of database
- Easier to test (no DB mocking needed)
- Could swap DB backend
- Core logic can be reused in CLI tools

---

### 2. **Error Handling with Typed Errors**

**Beancount Pattern:**
```python
ConfigError = collections.namedtuple('ConfigError', 'source message entry')
ValidationError = collections.namedtuple('ValidationError', 'source message entry')

def validate_entries(entries):
    errors = []
    for entry in entries:
        if not is_valid(entry):
            errors.append(ValidationError(
                source={'filename': entry.meta['filename'], 'lineno': entry.meta['lineno']},
                message="Entry is invalid",
                entry=entry
            ))
    return errors
```

**Castle Application:**
```python
from typing import NamedTuple, Optional

class CastleError(NamedTuple):
    """Base error type"""
    source: dict  # {'endpoint': '...', 'user_id': '...'}
    message: str
    entry: Optional[dict]  # Related entry data
    severity: str  # 'error', 'warning', 'info'
    code: str  # 'UNBALANCED_ENTRY', 'MISSING_ACCOUNT', etc.

class UnbalancedEntryError(NamedTuple):
    source: dict
    message: str
    entry: dict
    severity: str = 'error'
    code: str = 'UNBALANCED_ENTRY'
    total_debits: int
    total_credits: int
    difference: int

# Return errors from validation
async def validate_journal_entry(entry: CreateJournalEntry) -> list[CastleError]:
    errors = []

    total_debits = sum(line.debit for line in entry.lines)
    total_credits = sum(line.credit for line in entry.lines)

    if total_debits != total_credits:
        errors.append(UnbalancedEntryError(
            source={'created_by': entry.created_by},
            message=f"Entry does not balance: debits={total_debits}, credits={total_credits}",
            entry=entry.dict(),
            total_debits=total_debits,
            total_credits=total_credits,
            difference=total_debits - total_credits
        ))

    return errors
```

**Benefits:**
- Structured error information
- Easier to log and track
- Can return multiple errors at once
- Better error messages for users

---

## Implementation Roadmap

### Phase 1: Foundation (High Priority) ✅ COMPLETE
1. ✅ Switch from `float` to `Decimal` for fiat amounts
2. ✅ Add `meta` field to journal entries for audit trail
3. ✅ Add `flag` field for transaction status
4. ✅ Implement hierarchical account naming

### Phase 2: Reconciliation (High Priority) ✅ COMPLETE
5. ✅ Implement balance assertions
6. ✅ Add reconciliation API endpoints
7. ✅ Build reconciliation UI
8. ✅ Add automated daily balance checks

### Phase 3: Core Logic Refactoring (Medium Priority) - Improves code quality
9. Create `core/` module with pure accounting logic
10. Implement `CastleInventory` for position tracking
11. Move balance calculation to `core/balance.py`
12. Add comprehensive validation in `core/validation.py`

### Phase 4: Validation Plugins (Medium Priority) - Works better after Phase 3
13. Create plugin system architecture
14. Implement `check_balanced` plugin
15. Implement `check_receivables` plugin
16. Add plugin configuration UI

### Phase 5: Advanced Features (Low Priority)
17. Add tags and links to entries
18. Implement query language
19. Add lot tracking to inventory
20. Support multi-currency in single entry

---

## Key Takeaways

**What Beancount Does Right:**
1. ✅ Immutable data structures
2. ✅ Plugin architecture for extensibility
3. ✅ Rich metadata on every directive
4. ✅ Balance assertions for reconciliation
5. ✅ Hierarchical account naming
6. ✅ Use of Decimal for precision
7. ✅ Separation of core logic from I/O
8. ✅ Comprehensive validation

**What Castle Should Adopt First:**
1. **Decimal for fiat amounts** (prevent rounding errors)
2. **Meta field** (audit trail, source tracking)
3. **Flag field** (transaction status)
4. **Balance assertions** (reconciliation)
5. **Plugin system** (extensible validation)

**What Can Wait:**
- Query language (nice to have)
- Lot tracking (not needed for current use case)
- Full immutability (Pydantic models are fine for now)

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## Conclusion

Beancount's architecture is battle-tested for serious accounting work. By adopting these patterns, Castle can:
- Prevent financial calculation errors (Decimal)
- Support complex workflows (plugins)
- Build user trust (balance assertions, audit trail)
- Scale to larger deployments (clean architecture)
- Export to professional tools (hierarchical accounts)

The most important immediate change is switching from `float` to `Decimal` for fiat amounts. This alone will prevent serious bugs. The plugin system and balance assertions should follow shortly after as they're critical for financial correctness.