The scan no longer blindly grabs every movie/show section — it reads the
libraries you map, like music's 'pick your Music library'.
- GET /api/video/libraries: discover the active server's Movies/TV libraries
(Plex sections by type / Jellyfin views by CollectionType) + current
selection. POST: save {movies, tv} per server into video_settings.
- sources.py: _build_source(movies_lib, tv_lib) filters to the mapped library;
get_active_video_source() (used by the scanner) loads the saved selection;
list_video_libraries() lists them unfiltered for the UI. Falls back to all
libraries when nothing is mapped yet.
- VideoDatabase.get/set_library_selection (per-server). 6 tests added; 33 green.
