refactor(config): Set correct default sync interval and documentation

The default synchronization interval has been updated across the application to reflect a standard 30-minute delay (1800 seconds). This change updates both the source code configuration and the README documentation, ensuring that users rely on the accurate default value when no environment variable is provided.

- Updated SYNC_INTERVAL_SECONDS default in src/main.py
- Clarified single-run behavior in README.md

tests/test_main.py

@@ -142,6 +142,10 @@ class SynchronizationTests(unittest.TestCase):
             album_sync,
             "ALBUM_MODE",
             "root",
+        ), patch.object(
+            album_sync,
+            "SYNC_INTERVAL_SECONDS",
+            0,
         ), patch.object(
             album_sync,
             "check_configuration",