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/media.go

@@ -50,6 +50,48 @@ func (c *Client) SearchMovies(title string, year int) (int, error) {
 	return b.Results[0].Id, nil
 }
 
+func (c *Client) GetMovieExternalIds(tmdbId int) (string, error) {
+	reqUrl := util.JoinURL(c.apiURL, "movie", strconv.Itoa(tmdbId), "external_ids") + "?api_key=" + c.apiKey
+
+	resp, err := rek.Get(reqUrl, rek.Client(c.http))
+	if err != nil {
+		return "", fmt.Errorf("request movie external ids: %w", err)
+	}
+	defer resp.Body().Close()
+
+	if resp.StatusCode() != 200 {
+		return "", fmt.Errorf("validate movie external ids response: %s", resp.Status())
+	}
+
+	b := new(externalIdsResponse)
+	if err := json.NewDecoder(resp.Body()).Decode(b); err != nil {
+		return "", fmt.Errorf("decode movie external ids response: %w", err)
+	}
+
+	return b.ImdbId, nil
+}
+
+func (c *Client) GetShowExternalIds(tmdbId int) (string, error) {
+	reqUrl := util.JoinURL(c.apiURL, "tv", strconv.Itoa(tmdbId), "external_ids") + "?api_key=" + c.apiKey
+
+	resp, err := rek.Get(reqUrl, rek.Client(c.http))
+	if err != nil {
+		return "", fmt.Errorf("request show external ids: %w", err)
+	}
+	defer resp.Body().Close()
+
+	if resp.StatusCode() != 200 {
+		return "", fmt.Errorf("validate show external ids response: %s", resp.Status())
+	}
+
+	b := new(externalIdsResponse)
+	if err := json.NewDecoder(resp.Body()).Decode(b); err != nil {
+		return "", fmt.Errorf("decode show external ids response: %w", err)
+	}
+
+	return b.ImdbId, nil
+}
+
 func (c *Client) SearchShows(title string, year int) (int, error) {
 	vals := url.Values{
 		"api_key": []string{c.apiKey},