Replace hand-written TMDB types with openapi-fetch + generated schema

- Add `openapi-fetch` dependency; generate `lib/tmdb/schema.d.ts` from
  TMDB's OpenAPI spec and delete `lib/tmdb/types.ts`
- Rewrite `lib/tmdb/client.ts` to use the typed fetch client against
  the generated schema, exporting `TmdbMovieDetails`, `TmdbTvDetails`,
  `TmdbVideo`, and `TmdbGenre` directly
- Fix null-safety across services (`credits`, `availability`,
  `metadata`, `person`, `webhooks`) and call sites (explore page,
  search route, explore actions) to handle optional fields produced
  by the stricter generated types
- Update `metadata.ts` and `metadata.test.ts` to import shared types
  from `@/lib/tmdb/client` instead of the removed `types.ts`
- Exclude `lib/tmdb/schema.d.ts` from Biome linting

lib/services/metadata.test.ts

@@ -3,7 +3,7 @@ import type {
   TmdbMovieDetails,
   TmdbTvDetails,
   TmdbVideo,
-} from "@/lib/tmdb/types";
+} from "@/lib/tmdb/client";
 import {
   extractMovieContentRating,
   extractTvContentRating,