feat(library): video and image media entries + Library Video tab

[danieljtrujillo]

Adds first-class video/image support to the disk-backed library, the foundation for the VJ video library (cue persistence + overlays).

Backend:

• backend/modules/library/media.py: ffprobe/Pillow probing for dimensions, duration, and alpha (overlay-capable detection), plus poster-thumbnail rendering. Degrades gracefully when ffmpeg/ffprobe are absent (import still succeeds, no thumbnail).
• store: LibraryRecord carries kind/media_url/thumb_url/width/height/ has_alpha; import_media() stores the original untouched, probes it, and renders thumb.jpg. list_entries(kinds=...) filters by kind.
• router: GET /entries?kind=audio|video|image|media|all (default audio, so the audio library is unchanged), POST /import-media, GET /media/{id} (Range-capable), GET /media/{id}/thumb.
• 11 new tests (real PIL + ffmpeg fixtures); existing library suite green.

Frontend:

• LibraryEntry gains kind/mediaUrl/thumbUrl/width/height/hasAlpha.
• lib/mediaLibrary.ts: typed list/import/delete client for the media endpoints, kept separate from the audio StorageProvider abstraction.
• LibraryView: a Video sub-tab with a thumbnail grid (poster, duration, Alpha badge), multi-file Import button, drag-out with a stable URL, and remove. Audio tabs are untouched.