Season tiles showed the show poster because the wishlist had no season art. Now
stored + used: new video_wishlist.season_poster_url (SCHEMA_VERSION 11 + migration);
the get-modal captures each season's poster at add-time (owned -> /poster/season
proxy, tmdb -> direct); query_wishlist exposes season.poster_url; tiles render the
real season poster (falling back to show poster, then placeholder).
Backfill generalized: /wishlist/backfill-stills -> /wishlist/backfill-art fills
BOTH stills and season posters from the same cached tmdb_season call (it already
returns the season poster). The page fires it once when either is missing.
Tests updated/added: art backfill targets + season-poster set + endpoint. 104 passed.
e5cd5cf08c