feat: fallback to TMDB external_ids when Trakt rejects tmdbId

When Trakt returns a non-404 error (e.g. 403) for a tmdbId lookup,
fetch the imdbId from TMDB's /external_ids endpoint and retry the
Trakt call using the imdbId. Trakt often knows a movie/show by its
imdbId even when it does not recognise the tmdbId.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

media/tmdb/struct.go

@@ -11,3 +11,7 @@ type tvSearchResponse struct {
 		Id int `json:"id"`
 	} `json:"results"`
 }
+
+type externalIdsResponse struct {
+	ImdbId string `json:"imdb_id"`
+}