The Browse-by-genre tiles and the Collections browse tiles were flat color-hash
gradients. Give each a 2×2 collage of its top films' poster art, with the color hash
kept as the fallback so a group with no matching art (a show-only or empty collection)
never renders blank.
Backend: a shared `top_art_keys` windowed query returns, per key (a genre value or a
collection rating_key), the top few items' rating_keys — best-rated then newest, in one
query. Wired into the Discover genre tiles and the collections union endpoint; the client
renders the posters from the existing /api/plex/image/{key} endpoint, so no new image
plumbing. Frontend: a shared PosterTile component (collage + scrim + label, color-hash
fallback) replaces the two ad-hoc colored-tile implementations.
Verified live on localdev: genre tiles and collection tiles render real poster collages,
with the color fallback correctly kicking in for groups without art.
Gate: tsc/lint/knip/prettier clean, 423 vitest; backend ruff clean, 11 discover tests
(incl. a new genre-art assertion).
peter
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