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

README.md

@@ -37,6 +37,8 @@ Optional environment variables:
 - `ALBUM_SEPARATOR`: Separator for `relative` mode; defaults to ` - `
 - `PAGE_SIZE`: Immich search page size; defaults to `1000`
 - `DRY_RUN`: Set to `true` to log planned work without changing albums
+- `SYNC_INTERVAL_SECONDS`: Delay between runs; `0` runs once and exits.
+  Docker Compose defaults to `1800` seconds (30 minutes)
 
 Run the script after Immich has scanned the external library:
 
@@ -68,9 +70,10 @@ The photo directory does not need to be mounted into this container:
 `EXTERNAL_ROOT` is matched against the path recorded in Immich's
 `originalPath` field.
 
-The Compose service performs one synchronization and then exits. Run it after
-an external-library scan, or invoke it regularly with a scheduler such as
-cron.
+The Compose service stays running and synchronizes every 30 minutes. Change
+`SYNC_INTERVAL_SECONDS` in `.env` to use another interval. The delay starts
+after a synchronization finishes. Temporary API errors are logged and retried
+on the next scheduled run.
 
 The script only adds missing assets. It never removes assets from albums and
 never deletes albums.