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/show.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"strconv"
+	"strings"
 
 	"github.com/l3uddz/nabarr/media/trakt"
 )
@@ -21,7 +22,24 @@ func (c *Client) GetShowInfo(item *FeedItem) (*Item, error) {
 		if errors.Is(err, trakt.ErrItemNotFound) {
 			return nil, fmt.Errorf("trakt: get show: show with %sId %q: %w", mdp, mdi, ErrItemNotFound)
 		}
-		return nil, fmt.Errorf("trakt: get show: show with %sId %q: %w", mdp, mdi, err)
+
+		// fallback: if we queried by tmdbId and have TMDB configured, try fetching
+		// the imdbId from TMDB and retry Trakt — Trakt may know the show by imdbId
+		// even when it doesn't recognise the tmdbId (returns 403).
+		if c.tmdb != nil && mdp == "tmdb" {
+			if tmdbIdInt, convErr := strconv.Atoi(mdi); convErr == nil {
+				if imdbId, extErr := c.tmdb.GetShowExternalIds(tmdbIdInt); extErr == nil && strings.HasPrefix(imdbId, "tt") {
+					t, err = c.trakt.GetShow("imdb", imdbId)
+				}
+			}
+		}
+
+		if err != nil {
+			if errors.Is(err, trakt.ErrItemNotFound) {
+				return nil, fmt.Errorf("trakt: get show: show with %sId %q: %w", mdp, mdi, ErrItemNotFound)
+			}
+			return nil, fmt.Errorf("trakt: get show: show with %sId %q: %w", mdp, mdi, err)
+		}
 	}
 
 	// transform trakt info to MediaItem