Root cause: season posters / episode art backfill happen during a show's TMDB
*match*, but already-matched shows never re-run ('Retry all failed' only resets
not_found/error), so existing libraries never got the art.
Fix (Boulder's idea): fetch-on-view + cache. When a show detail opens and any
season lacks a poster, the page calls POST /detail/show/<id>/refresh-art →
engine.refresh_show_art re-fetches /tv/<id> via the TMDB client and backfills
season posters + episode art gap-only, regardless of match status. Cached, so
it's a one-time cost per show; runs once per view; re-renders when done.
Seam tests: refresh_show_art backfills a MATCHED show's seasons, needs TMDB
configured, show_match_info, route registered.
986059626f