Fix API controller tests: 404 handling, JSON encoding, media filtering

- Add FallbackController for proper 404 JSON responses on missing resources
- Add non-bang get_source/1 and get_media_item/1 for safe lookups
- Fix Task JSON encoder: remove Repo.preload call (not available in encoder scope),
  handle NotLoaded associations by returning nil
- Fix media item index: use list_media_items_for_source/1 instead of
  list_pending_media_items_for/1 which had overly restrictive filtering
- Add missing extras_directory to test config
- All 1007 tests pass

config/test.exs

@@ -6,6 +6,7 @@ config :pinchflat,
   apprise_executable: Path.join([File.cwd!(), "/test/support/scripts/yt-dlp-mocks/repeater.sh"]),
   media_directory: Path.join([System.tmp_dir!(), "test", "media"]),
   metadata_directory: Path.join([System.tmp_dir!(), "test", "metadata"]),
+  extras_directory: Path.join([System.tmp_dir!(), "test", "extras"]),
   tmpfile_directory: Path.join([System.tmp_dir!(), "test", "tmpfiles"]),
   file_watcher_poll_interval: 50