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

tests/test_main.py

@@ -174,6 +174,50 @@ class SynchronizationTests(unittest.TestCase):
             ["asset-one", "asset-two"],
         )
 
+    def test_continuous_mode_waits_and_runs_again(self) -> None:
+        with patch.object(
+            album_sync,
+            "SYNC_INTERVAL_SECONDS",
+            1800,
+        ), patch.object(
+            album_sync,
+            "check_configuration",
+        ), patch.object(
+            album_sync,
+            "synchronize",
+            side_effect=[None, KeyboardInterrupt],
+        ) as synchronize, patch.object(
+            album_sync.time,
+            "sleep",
+        ) as sleep:
+            with self.assertRaises(KeyboardInterrupt):
+                album_sync.main()
+
+        self.assertEqual(synchronize.call_count, 2)
+        sleep.assert_called_once_with(1800)
+
+    def test_continuous_mode_retries_after_sync_error(self) -> None:
+        with patch.object(
+            album_sync,
+            "SYNC_INTERVAL_SECONDS",
+            1800,
+        ), patch.object(
+            album_sync,
+            "check_configuration",
+        ), patch.object(
+            album_sync,
+            "synchronize",
+            side_effect=[RuntimeError("temporary"), KeyboardInterrupt],
+        ) as synchronize, patch.object(
+            album_sync.time,
+            "sleep",
+        ) as sleep:
+            with self.assertRaises(KeyboardInterrupt):
+                album_sync.main()
+
+        self.assertEqual(synchronize.call_count, 2)
+        sleep.assert_called_once_with(1800)
+
 
 if __name__ == "__main__":
     unittest.main()