[v1] Collection membership on media detail pages #230

Summary

On a movie/series detail page, show the collections that item belongs to, with links to browse each collection's full item list. Currently there's no way to navigate from an item to its collections or see what else is in them.

User value / why v1

Collections are a core discovery mechanism (franchises, studios, genres, curated lists). When viewing Dune (1984), the user should see it's in "Dune Saga" / "Sci-Fi" collections and jump to those to find related titles. Without this, collections are only reachable from the library browse view — disconnected from the items they contain.

API surface

• GET /api/v2/items/{id}/collections — list collections containing this item (id, title, poster_url, item_count). Or extend the existing item detail response with a collections array.
• Existing collection detail endpoint already lists items; this just needs the reverse lookup.

Client impact

• Web: new "Collections" section/row on the detail page with poster chips linking to each collection view
• Android/Apple: same section; "None" if clients defer the UI, but API should still ship

Rough size

S (server-side): reverse lookup on the existing item↔collection mapping + include in detail response.

AI harness

none

AI tool(s)

none

AI model(s)

n/a

AI involvement

No AI used

Independent or adversarial review

n/a