Backend for the upcoming TV/movie detail pages, isolated to video.db:
- show_detail(id): show + seasons->episodes tree with owned/total roll-ups
(season 0 -> 'Specials', missing-season-row episodes still grouped).
- movie_detail(id): movie + owned flag + best media-file (resolution/quality).
- get_art_ref generalizes the poster ref to poster|backdrop; new
/api/video/backdrop/<kind>/<id> streams the hero art server-side (Jellyfin
Backdrop vs Primary handled).
- /api/video/detail/{show,movie}/<id> endpoints.
Seam tests for the tree roll-ups, owned/file, art ref, and both endpoints.
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