feat: mark missing files as pending and auto-mount series volume when Sonarr is active

Add "pending" flag to cached() error response for missing files to enable download status detection. Update docker-compose to always mount series volume with fallback default path instead of requiring manual uncomment. Update README to clarify that SERIES_MEDIA_VOLUME must point to actual host path when Sonarr is enabled.

docker-compose.yml

@@ -10,8 +10,8 @@ services:
     volumes:
       # Host-Pfad deiner Film-Library -> interner Dashboard-Pfad
       - ${FILM_MEDIA_VOLUME}:/media/filme:ro
-      # Sonarr aktivieren, wenn SERIES_MEDIA_VOLUME gesetzt ist:
-      # - ${SERIES_MEDIA_VOLUME}:/media/serien:ro
+      # Falls Sonarr verwendet wird, muss SERIES_MEDIA_VOLUME auf die Serien-Library zeigen.
+      - ${SERIES_MEDIA_VOLUME:-/nesflix/serien}:/media/serien:ro
       - ./data:/data
     networks:
       - media