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_test.go

@@ -94,3 +94,33 @@ func TestResolveURLBase(t *testing.T) {
 		t.Errorf("no url base: %q", got)
 	}
 }
+
+func TestPosterFallsBackToTheProviderWhenTheInstanceHasNoCopy(t *testing.T) {
+	var remoteKey string
+	remote := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		remoteKey = r.Header.Get("X-Api-Key")
+		w.Write([]byte("REMOTE"))
+	}))
+	defer remote.Close()
+	instance := httptest.NewServer(http.NotFoundHandler()) // MediaCover has nothing cached
+	defer instance.Close()
+
+	c := NewClient(instance.URL, "secret")
+	_, fetch, ok := c.Poster([]Image{{CoverType: "poster", URL: "/MediaCover/9/poster.jpg", RemoteURL: remote.URL + "/p.jpg"}}, true)
+	if !ok {
+		t.Fatal("no poster")
+	}
+	data, err := fetch(context.Background())
+	if err != nil || string(data) != "REMOTE" {
+		t.Fatalf("data=%q err=%v", data, err)
+	}
+	if remoteKey != "" {
+		t.Errorf("the API key went to the provider: %q", remoteKey)
+	}
+
+	// Without a remote address the local failure is reported.
+	_, fetch, _ = c.Poster([]Image{{CoverType: "poster", URL: "/MediaCover/9/poster.jpg"}}, true)
+	if _, err := fetch(context.Background()); err == nil {
+		t.Error("expected the local error")
+	}
+}