feat(sync): enable scheduled asset synchronization mode

This update introduces a persistent background synchronization mode, allowing the external assets container to run continuously and periodically sync albums instead of executing once and exiting. The system now checks a configurable interval, retries on temporary failures, and manages its lifecycle using `time.sleep`.

- Added SYNC_INTERVAL_SECONDS configuration for scheduling
- Updated compose service to restart automatically and loop indefinitely
- Implemented continuous run logic in main function and added corresponding unit tests

.env.example

@@ -11,3 +11,6 @@ PAGE_SIZE=1000
 
 # Start with true to verify the planned album assignments without changes.
 DRY_RUN=true
+
+# Keep the container running and synchronize every 30 minutes.
+SYNC_INTERVAL_SECONDS=1800