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

src/main.py

@@ -2,6 +2,7 @@ import logging
 import os
 import posixpath
 import sys
+import time
 from pathlib import PurePosixPath
 from typing import Any
 
@@ -31,6 +32,9 @@ ALBUM_MODE = os.environ.get("ALBUM_MODE", "root").lower()
 ALBUM_SEPARATOR = os.environ.get("ALBUM_SEPARATOR", " - ")
 PAGE_SIZE = int(os.environ.get("PAGE_SIZE", "1000"))
 DRY_RUN = os.environ.get("DRY_RUN", "false").lower() == "true"
+SYNC_INTERVAL_SECONDS = int(
+    os.environ.get("SYNC_INTERVAL_SECONDS", "0")
+)
 
 logging.basicConfig(
     level=logging.INFO,
@@ -73,6 +77,11 @@ def check_configuration() -> None:
     if not EXTERNAL_ROOT.startswith("/"):
         raise RuntimeError("EXTERNAL_ROOT must be an absolute path")
 
+    if SYNC_INTERVAL_SECONDS < 0:
+        raise RuntimeError(
+            "SYNC_INTERVAL_SECONDS must be zero or greater"
+        )
+
 
 def api_request(
     method: str,
@@ -232,9 +241,7 @@ def get_external_assets() -> list[dict[str, Any]]:
     return assets
 
 
-def main() -> int:
-    check_configuration()
-
+def synchronize() -> None:
     existing_albums = get_albums()
     assets = get_external_assets()
 
@@ -292,6 +299,30 @@ def main() -> int:
             )
 
     logging.info("Synchronization completed")
+
+
+def main() -> int:
+    check_configuration()
+
+    while True:
+        try:
+            synchronize()
+        except Exception:
+            if SYNC_INTERVAL_SECONDS == 0:
+                raise
+            logging.exception(
+                "Synchronization failed; the next run will retry"
+            )
+
+        if SYNC_INTERVAL_SECONDS == 0:
+            break
+
+        logging.info(
+            "Next synchronization in %d seconds",
+            SYNC_INTERVAL_SECONDS,
+        )
+        time.sleep(SYNC_INTERVAL_SECONDS)
+
     return 0