The media server rarely has distinct per-season art, so season cards fell back to
a gradient. TMDB's show detail carries a poster_path per season — the show worker
now returns those, and enrichment_apply backfills seasons.poster_url for seasons
the server left without art (gap-only, never clobbers server art). The image
proxy streams a stored full URL (TMDB) directly vs. proxying a server path.
Seam tests: TMDB returns season posters, backfill fills only missing seasons.
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