Entity-less media search for the Library add flow.

Proxies TMDB (movie/tv) or Google Books (book) and returns enriched candidates matching the shape of /library/{id}/search-tmdb and /library/{id}/search-google-books so iOS/web can reuse existing Codable/TS types. No tenant data is touched — auth gate only. Underlying services use sync httpx.get(), so every call is wrapped in asyncio.to_thread() per app/CLAUDE.md "Async Endpoints — NEVER Block the Event Loop."