Implement hybrid approach for balance assertions

Balance assertions now use a hybrid architecture where Beancount is the source
of truth for validation, while Castle stores metadata for UI convenience.

Backend changes:
- Add format_balance() function to beancount_format.py for formatting balance directives
- Update POST /api/v1/assertions to write balance directive to Beancount first (via Fava)
- Store metadata in Castle DB (created_by, tolerance, notes) for UI features
- Validate assertions immediately by querying Fava for actual balance

Frontend changes:
- Update dialog description to explain Beancount validation
- Update button tooltip to clarify balance assertions are written to Beancount
- Update empty state message to mention Beancount checkpoints

Benefits:
- Single source of truth (Beancount ledger file)
- Automatic validation by Beancount
- Best of both worlds: robust validation + friendly UI

See misc-docs/BALANCE-ASSERTIONS-HYBRID-APPROACH.md for full documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

beancount_format.py

@@ -104,6 +104,36 @@ def format_transaction(
     }
 
 
+def format_balance(
+    date_val: date,
+    account: str,
+    amount: int,
+    currency: str = "SATS"
+) -> str:
+    """
+    Format a balance assertion directive for Beancount.
+
+    Balance assertions verify that an account has an expected balance on a specific date.
+    They are checked automatically by Beancount when the file is loaded.
+
+    Args:
+        date_val: Date of the balance assertion
+        account: Account name (e.g., "Assets:Bitcoin:Lightning")
+        amount: Expected balance amount
+        currency: Currency code (default: "SATS")
+
+    Returns:
+        Beancount balance directive as a string
+
+    Example:
+        >>> format_balance(date(2025, 11, 10), "Assets:Bitcoin:Lightning", 1500000, "SATS")
+        '2025-11-10 balance Assets:Bitcoin:Lightning  1500000 SATS'
+    """
+    date_str = date_val.strftime('%Y-%m-%d')
+    # Two spaces between account and amount (Beancount convention)
+    return f"{date_str} balance {account}  {amount} {currency}"
+
+
 def format_posting_with_cost(
     account: str,
     amount_sats: int,