feat: add background library sync thread with configurable interval to eliminate browser dependency for automatic scanning

Add SYNC_INTERVAL_SECONDS environment variable with default 1800 (30 minutes). Add library_cache global dict with rows/series/updated_at/error/ready fields and library_cache_lock for thread-safe access. Extract refresh_library_cache() function to update Radarr/Sonarr data with error handling that preserves last good snapshot during outages. Add background_library_sync() daemon

docker-compose.yml

@@ -20,6 +20,8 @@ services:
     environment:
       NVIDIA_VISIBLE_DEVICES: ${NVIDIA_VISIBLE_DEVICES:-all}
       NVIDIA_DRIVER_CAPABILITIES: ${NVIDIA_DRIVER_CAPABILITIES:-compute,utility}
+      # Serverseitiger Bibliotheks-Sync; 1800 = alle 30 Minuten, 0 = nur beim Start.
+      SYNC_INTERVAL_SECONDS: ${SYNC_INTERVAL_SECONDS:-1800}
     volumes:
       # Host-Pfad deiner Film-Library -> interner Dashboard-Pfad
       - ${FILM_MEDIA_VOLUME}:/media/filme:ro