fix(tmdb): fall back to original language for missing trailers too

TMDB language-filters videos the same way it filters overviews: a
Russian-only title has its trailer tagged iso_639_1=ru, so an en-US
request returns no trailer — the trailer only appeared when the user
switched the app language. The original-language fallback previously
triggered on an empty overview only, so titles that had a translated
overview but a language-only trailer never got one.

detailsFallbackLanguage now also triggers when the payload has no usable
YouTube trailer, and fillDetailsFromFallback fills the overview and the
trailer independently — a present app-language overview is kept while the
trailer is pulled from the original-language payload. The fallback fetch
stays best-effort and cached per language.

Tests: extended tmdb-language-fallback.spec.ts (trailer-missing trigger,
independent overview/trailer fill, YouTube-only guard). Docs updated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/architecture/tmdb-metadata-enrichment.md

@@ -139,13 +139,15 @@ The `language` param derives from the app language setting
 (`Language` enum → TMDB code, e.g. `de` → `de-DE`); cache rows are keyed per
 language, so switching the app language re-fetches localized metadata.
 
-TMDB does NOT fall back on missing translations — a Russian-only series
-returns empty overviews for `en-US`. When the app-language payload has no
-overview, the enrichment refetches once in the content's
-`original_language` and fills only the missing text
-(`tmdb-language-fallback.ts`): the details overview, and — via the same
-rule in `TmdbSeasonService` when a season payload carries no usable text —
-the season overview and per-episode names/overviews. Genres, credits and
+TMDB language-filters both text AND videos — a Russian-only title returns
+an empty overview and no trailer for `en-US` (its trailer is tagged
+`iso_639_1=ru`). When the app-language payload is missing either, the
+enrichment refetches once in the content's `original_language` and fills
+only the missing fields (`tmdb-language-fallback.ts`): the details
+overview and/or trailer (each independently, so a present app-language
+overview is kept while the trailer is filled), and — via the same rule in
+`TmdbSeasonService` when a season payload carries no usable text — the
+season overview and per-episode names/overviews. Genres, credits and
 artwork stay in the app language; both language rows land in the cache.
 
 Trailers embed via `https://www.youtube-nocookie.com/embed/…`. YouTube