Remove GoCardless fallback from /accounts endpoints

- Remove GoCardless API calls from /api/v1/accounts and /api/v1/accounts/{account_id}
- Accounts endpoints now rely exclusively on database data
- Return 404 for accounts not found in database
- Update tests to mock database service instead of GoCardless API
- Remove unused GoCardless imports from transactions routes
- Preserve GoCardless usage in sync process and /banks endpoints
- Fix code formatting and remove unused imports

leggend/services/sync_service.py

@@ -55,6 +55,10 @@ class SyncService:
                         account_id
                     )
 
+                    # Persist account details to database
+                    if account_details:
+                        await self.database.persist_account_details(account_details)
+
                     # Get and save balances
                     balances = await self.gocardless.get_account_balances(account_id)
                     if balances: