fix(artwork): register the GIF decoder in core/artwork

The deleted artwork.go carried blank imports for image/gif and
x/image/webp. WebP came back via resize.go's gen2brain/webp, which
self-registers, but GIF did not: core/artwork claims GIF support in
mimeForFormat and extForMime while relying on an unrelated server
package to have imported the decoder.

The guard lives in the e2e suite because that test binary has no other
image/gif importer; a spec in core/artwork would pass regardless, since
animation_test.go imports the package non-blank.

core/artwork/e2e/acquire_serve_test.go

@@ -229,6 +229,20 @@ var _ = Describe("Acquisition → serve loop", func() {
 		Expect(art.BlurHash).ToNot(BeEmpty())
 	})
 
+	It("acquires GIF artwork, whose decoder only core/artwork's blank import registers", func() {
+		writeUploadedImage(consts.EntityRadio, "station.gif", gifFixture)
+		radioRepo.Data["ra1"] = &model.Radio{ID: "ra1", Name: "Station", UploadedImage: "station.gif"}
+		worker.Bump("ra", "ra1")
+		runWorkerUntil(ctx, worker, itemFound(model.KindRadioArtwork, "ra1"))
+
+		ia, err := artRepo.GetItemArtwork(model.KindRadioArtwork, "ra1", model.ImageTypePrimary)
+		Expect(err).ToNot(HaveOccurred())
+		art, err := artRepo.GetImage(ia.Hash)
+		Expect(err).ToNot(HaveOccurred())
+		Expect(art.Mime).To(Equal("image/gif"))
+		Expect(art.Width).To(BeNumerically("==", 4))
+	})
+
 	It("deduplicates byte-identical art across entities onto one image row", func() {
 		folderRepo.result = []model.Folder{{Path: albumFolderPath, ImageFiles: []string{"cover.jpg"}}}
 		albumRepo.SetData(model.Albums{
@@ -299,3 +313,12 @@ func mustGet(img *artwork.Image, err error) *artwork.Image {
 	Expect(err).ToNot(HaveOccurred())
 	return img
 }
+
+// gifFixture is a 4x4 GIF held as raw bytes on purpose: encoding one would import image/gif into
+// this test binary and register the decoder, masking the production import the spec above guards.
+var gifFixture = []byte{
+	0x47, 0x49, 0x46, 0x38, 0x39, 0x61, 0x04, 0x00, 0x04, 0x00, 0x80, 0x00,
+	0x00, 0x2e, 0x86, 0xc1, 0xf4, 0xd0, 0x3f, 0x2c, 0x00, 0x00, 0x00, 0x00,
+	0x04, 0x00, 0x04, 0x00, 0x00, 0x02, 0x05, 0x44, 0x7c, 0x67, 0xb8, 0x05,
+	0x00, 0x3b,
+}

core/artwork/processor.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"image"
 	"image/draw"
+	_ "image/gif" // the only artwork format with no other importer in this package
 	"io"
 	"time"