Factor out `backend/routes/tmdb.ts`: extract `buildListEndpoint` helper

[arediss]

Context

packages/backend/src/routes/tmdb.ts = 443 lines, 21 handlers. 13 handlers follow the exact same copy-paste pattern:

getLang(request) → TMDB service → flagNsfwFromDb → return { results, nsfwTmdbIds }

Clone handlers:

• /trending (l.121)
• /movies/popular (l.132)
• /tv/popular (l.143)
• /movies/upcoming (l.165)
• /tv/trending-anime (l.154)
• /tv/top_rated (l.198)
• /tv/airing_today (l.209)
• /tv/on_the_air (l.220)
• /search (l.231)
• /movie/:id/recommendations (l.288)
• /tv/:id/recommendations (l.301)
• /discover/mixed (l.379)
• /discover/:mediaType/genre/:genreId (l.324)

Scope

• Extract a buildListEndpoint(service, options?) helper to factor out the 13 handlers
• Group by subdomain: tmdb/list.ts, tmdb/details.ts, tmdb/discover.ts, tmdb/genres.ts
• Add unit tests on the extracted helper (NSFW filtering, language, pagination)

Success criteria

• tmdb.ts reduced to pure orchestration (< 150 lines)
• Adding a new list endpoint = 1 helper call

Parent: #145