PHASE 2: Implements balance assertions for reconciliation

Adds balance assertion functionality to enable admins to verify accounting accuracy.

This includes:
- A new `balance_assertions` table in the database
- CRUD operations for balance assertions (create, get, list, check, delete)
- API endpoints for managing balance assertions (admin only)
- UI elements for creating, viewing, and re-checking assertions

Also, reorders the implementation roadmap in the documentation to reflect better the dependencies between phases.

models.py

@@ -179,3 +179,41 @@ class RecordPayment(BaseModel):
     """Record a payment"""
 
     payment_hash: str
+
+
+class AssertionStatus(str, Enum):
+    """Status of a balance assertion"""
+    PENDING = "pending"    # Not yet checked
+    PASSED = "passed"      # Assertion passed (balance matches)
+    FAILED = "failed"      # Assertion failed (balance mismatch)
+
+
+class BalanceAssertion(BaseModel):
+    """Assert expected balance at a specific date for reconciliation"""
+    id: str
+    date: datetime
+    account_id: str
+    expected_balance_sats: int  # Expected balance in satoshis
+    expected_balance_fiat: Optional[Decimal] = None  # Optional fiat balance
+    fiat_currency: Optional[str] = None  # Currency for fiat balance (EUR, USD, etc.)
+    tolerance_sats: int = 0  # Allow +/- this much difference in sats
+    tolerance_fiat: Decimal = Decimal("0")  # Allow +/- this much difference in fiat
+    checked_balance_sats: Optional[int] = None  # Actual balance found
+    checked_balance_fiat: Optional[Decimal] = None  # Actual fiat balance found
+    difference_sats: Optional[int] = None  # Difference in sats
+    difference_fiat: Optional[Decimal] = None  # Difference in fiat
+    status: AssertionStatus = AssertionStatus.PENDING
+    created_by: str
+    created_at: datetime
+    checked_at: Optional[datetime] = None
+
+
+class CreateBalanceAssertion(BaseModel):
+    """Create a balance assertion"""
+    account_id: str
+    date: Optional[datetime] = None  # If None, use current time
+    expected_balance_sats: int
+    expected_balance_fiat: Optional[Decimal] = None
+    fiat_currency: Optional[str] = None
+    tolerance_sats: int = 0
+    tolerance_fiat: Decimal = Decimal("0")