REFACTOR Migrates to single 'amount' field for transactions

Refactors the data model to use a single 'amount' field for journal entry lines, aligning with the Beancount approach.
This simplifies the model, enhances compatibility, and eliminates invalid states.

Includes a database migration to convert existing debit/credit columns to the new 'amount' field.

Updates balance calculation logic to utilize the new amount field for improved accuracy and efficiency.

tasks.py

@@ -207,15 +207,13 @@ async def on_invoice_paid(payment: Payment) -> None:
             lines=[
                 CreateEntryLine(
                     account_id=lightning_account.id,
-                    debit=amount_sats,
-                    credit=0,
+                    amount=amount_sats,  # Positive = debit (asset increase)
                     description="Lightning payment received",
                     metadata=line_metadata,
                 ),
                 CreateEntryLine(
                     account_id=user_receivable.id,
-                    debit=0,
-                    credit=amount_sats,
+                    amount=-amount_sats,  # Negative = credit (asset decrease - receivable settled)
                     description="Payment applied to balance",
                     metadata=line_metadata,
                 ),