The 'no episode images' was data, not a bug: existing episode rows predate
still-capture (81 rows, 0 stills). New /wishlist/backfill-stills fills them
cheaply — one cached tmdb_season call per (show, season), updating only rows
that lack a still. The show tab fires it once automatically when it sees missing
stills, then reloads so the images pop in.
DB: wishlist_still_backfill_targets + set_wishlist_still (won't clobber existing).
Tests: +2. Suites green.
