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

compose.yaml

@@ -4,7 +4,7 @@ services:
       context: .
     image: immich-extern-to-album:local
     init: true
-    restart: "no"
+    restart: unless-stopped
     environment:
       IMMICH_URL: ${IMMICH_URL:?Set IMMICH_URL in .env}
       IMMICH_API_KEY: ${IMMICH_API_KEY}
@@ -14,6 +14,7 @@ services:
       ALBUM_SEPARATOR: "${ALBUM_SEPARATOR:- - }"
       PAGE_SIZE: ${PAGE_SIZE:-1000}
       DRY_RUN: ${DRY_RUN:-false}
+      SYNC_INTERVAL_SECONDS: ${SYNC_INTERVAL_SECONDS:-1800}
     extra_hosts:
       - "host.docker.internal:host-gateway"
     read_only: true