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/api/routes/transactions.py

@@ -5,11 +5,9 @@ from loguru import logger
 
 from leggend.api.models.common import APIResponse
 from leggend.api.models.accounts import Transaction, TransactionSummary
-from leggend.services.gocardless_service import GoCardlessService
 from leggend.services.database_service import DatabaseService
 
 router = APIRouter()
-gocardless_service = GoCardlessService()
 database_service = DatabaseService()