Adds background task for invoice processing

Implements a background task that listens for paid invoices
and automatically records them in the accounting system. This
ensures payments are captured even if the user closes their
browser before the client-side polling detects the payment.

Introduces a new `get_journal_entry_by_reference` function to
improve idempotency when recording payments.

__init__.py

@@ -1,6 +1,10 @@
+import asyncio
+
 from fastapi import APIRouter
+from loguru import logger
 
 from .crud import db
+from .tasks import wait_for_paid_invoices
 from .views import castle_generic_router
 from .views_api import castle_api_router
 
@@ -15,4 +19,11 @@ castle_static_files = [
     }
 ]
 
-__all__ = ["castle_ext", "castle_static_files", "db"]
+
+def castle_start():
+    """Initialize Castle extension background tasks"""
+    logger.info("Starting Castle accounting extension background tasks")
+    asyncio.create_task(wait_for_paid_invoices())
+
+
+__all__ = ["castle_ext", "castle_static_files", "db", "castle_start"]