Posters: fall back to the provider's image when the instance has no copy

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

internal/arr/image.go

@@ -25,7 +25,7 @@ func (c *Client) Poster(images []Image, inLibrary bool) (key string, fetch func(
 			continue
 		}
 		if inLibrary && img.URL != "" {
-			local := img.URL
+			local, remote := img.URL, img.RemoteURL
 			base, _ := c.settings()
 			return base + "|" + local, func(ctx context.Context) ([]byte, error) {
 				// MediaCover has poster-250.jpg next to poster.jpg; fall back to the original.
@@ -34,7 +34,15 @@ func (c *Client) Poster(images []Image, inLibrary bool) (key string, fetch func(
 						return data, nil
 					}
 				}
-				return c.GetBytes(ctx, local)
+				data, err := c.GetBytes(ctx, local)
+				if err != nil && remote != "" {
+					// The instance has not cached (or has lost) its copy: use the
+					// metadata provider's, as for titles that are not in the library.
+					if remoteData, remoteErr := FetchURL(ctx, remote); remoteErr == nil {
+						return remoteData, nil
+					}
+				}
+				return data, err
 			}, true
 		}
 		if img.RemoteURL != "" {