fix: retry failed discover artwork

internal/service/image_proxy_test.go

@@ -132,6 +132,52 @@ func TestImageProxyCachesFailedRemoteImageFetch(t *testing.T) {
 	}
 }
 
+func TestImageProxyRemoveFailedAllowsRetry(t *testing.T) {
+	var calls int32
+	proxy := NewImageProxy(&config.Config{Cache: config.CacheConfig{CacheDir: filepath.Join(t.TempDir(), "cache")}}, zap.NewNop())
+	proxy.client = &http.Client{Transport: imageRoundTripFunc(func(req *http.Request) (*http.Response, error) {
+		call := atomic.AddInt32(&calls, 1)
+		if call == 1 {
+			return &http.Response{
+				StatusCode: http.StatusBadGateway,
+				Status:     "502 Bad Gateway",
+				Header:     make(http.Header),
+				Body:       io.NopCloser(strings.NewReader("upstream unavailable")),
+				Request:    req,
+			}, nil
+		}
+		return &http.Response{
+			StatusCode: http.StatusOK,
+			Status:     "200 OK",
+			Header:     http.Header{"Content-Type": []string{"image/jpeg"}},
+			Body:       io.NopCloser(strings.NewReader("poster-bytes")),
+			Request:    req,
+		}, nil
+	})}
+
+	raw := "https://image.tmdb.org/t/p/w500/retry-poster.jpg"
+	rec := httptest.NewRecorder()
+	if err := proxy.Serve(t.Context(), rec, httptest.NewRequest(http.MethodGet, "/api/img", nil), raw); err != nil {
+		t.Fatal(err)
+	}
+	if rec.Body.Len() != len(transparent1x1PNG) {
+		t.Fatalf("first body length = %d, want placeholder %d", rec.Body.Len(), len(transparent1x1PNG))
+	}
+	if err := proxy.RemoveFailed(raw); err != nil {
+		t.Fatal(err)
+	}
+	rec = httptest.NewRecorder()
+	if err := proxy.Serve(t.Context(), rec, httptest.NewRequest(http.MethodGet, "/api/img?v=retry", nil), raw); err != nil {
+		t.Fatal(err)
+	}
+	if got := rec.Body.String(); got != "poster-bytes" {
+		t.Fatalf("retried body = %q, want poster bytes", got)
+	}
+	if got := atomic.LoadInt32(&calls); got != 2 {
+		t.Fatalf("upstream calls = %d, want 2 after retry", got)
+	}
+}
+
 func TestImageProxyCachesCloudResolvedImage(t *testing.T) {
 	var calls int32
 	proxy := NewImageProxy(&config.Config{Cache: config.CacheConfig{CacheDir: filepath.Join(t.TempDir(), "cache")}}, zap.NewNop())