Real-image posters: scale to the box's pixel size

Ghostty shows an image smaller than its placement at its own size, so
library posters (Radarr's poster-500 is 333x500, not 500x750) came out
smaller than the 426x640 remote ones. Images are now scaled (bilinear)
to exactly cols*cellWidth x rows*cellHeight pixels before upload, and
library titles use the original poster for real images. The disk cache
stores posters as decoded (JPEG) instead of the possibly large originals.

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

internal/arr/image_test.go

@@ -24,7 +24,7 @@ func TestScaledVariant(t *testing.T) {
 	}
 }
 
-func TestLargePosterUsesThe500Variant(t *testing.T) {
+func TestLargePosterUsesTheOriginal(t *testing.T) {
 	var paths []string
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		paths = append(paths, r.URL.Path)
@@ -39,7 +39,7 @@ func TestLargePosterUsesThe500Variant(t *testing.T) {
 	if _, err := fetch(context.Background(), false); err != nil {
 		t.Fatal(err)
 	}
-	if strings.Join(paths, " ") != "/MediaCover/1/poster-500.jpg /MediaCover/1/poster-250.jpg" {
+	if strings.Join(paths, " ") != "/MediaCover/1/poster.jpg /MediaCover/1/poster-250.jpg" {
 		t.Errorf("requests = %v", paths)
 	}
 }