PHASE 2: Fixes balance assertion creation and validation

Improves balance assertion creation by handling Decimal types correctly for database insertion.
It also fixes a validation issue in the UI where the expected balance was not correctly validated as a required field and initializes the value to 0.

crud.py

@@ -796,7 +796,31 @@ async def create_balance_assertion(
         created_at=datetime.now(),
     )
 
-    await db.insert("balance_assertions", assertion, convert_decimal=True)
+    # Manually insert with Decimal fields converted to strings
+    await db.execute(
+        """
+        INSERT INTO balance_assertions (
+            id, date, account_id, expected_balance_sats, expected_balance_fiat,
+            fiat_currency, tolerance_sats, tolerance_fiat, status, created_by, created_at
+        ) VALUES (
+            :id, :date, :account_id, :expected_balance_sats, :expected_balance_fiat,
+            :fiat_currency, :tolerance_sats, :tolerance_fiat, :status, :created_by, :created_at
+        )
+        """,
+        {
+            "id": assertion.id,
+            "date": assertion.date,
+            "account_id": assertion.account_id,
+            "expected_balance_sats": assertion.expected_balance_sats,
+            "expected_balance_fiat": str(assertion.expected_balance_fiat) if assertion.expected_balance_fiat else None,
+            "fiat_currency": assertion.fiat_currency,
+            "tolerance_sats": assertion.tolerance_sats,
+            "tolerance_fiat": str(assertion.tolerance_fiat),
+            "status": assertion.status.value,
+            "created_by": assertion.created_by,
+            "created_at": assertion.created_at,
+        },
+    )
     return assertion