Trakt lists: keep the Radarr/Sonarr lookups of their titles on disk

Library membership is asked every time via movie?tmdbId= / series?tvdbId=;
only the lookups of titles not in the library are stored (3 days).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/sonarr/sonarr.go

@@ -111,6 +111,19 @@ func (c *Client) Series(ctx context.Context) ([]Series, error) {
 	return series, err
 }
 
+// SeriesByTVDB returns the library series with the TVDB id, ok=false if the
+// library has none. Unlike a lookup it does not ask the metadata provider.
+func (c *Client) SeriesByTVDB(ctx context.Context, tvdbID int) (Series, bool, error) {
+	var series []Series
+	err := c.Do(ctx, http.MethodGet, "series", url.Values{"tvdbId": {strconv.Itoa(tvdbID)}}, nil, &series)
+	for _, s := range series { // checked: a version that ignores the filter sends the whole library
+		if s.TVDBID == tvdbID && s.InLibrary() {
+			return s, true, err
+		}
+	}
+	return Series{}, false, err
+}
+
 // AddOptions are the choices from the add form.
 type AddOptions struct {
 	QualityProfileID int