fix(artwork): version the resized cache key to backfill blurhashes on upgrade

Resized entries cached by a pre-blurhash version serve straight from the cache:
the resized reader never runs, the original is never read, and the tee never gets
a chance to compute a hash — clients that only request sized images (Jellyfin
maxwidth) would keep receiving the fake blurhash indefinitely for those items.

Versioning the resized cache key makes every post-upgrade sized request miss and
refill. The refill pulls the original through Get, which usually hits the cached
original (original keys are unchanged), so the backfill costs one decode+re-encode
per resized variant with no source or external-provider I/O. Orphaned entries are
evicted by the cache's LRU as usual.

core/artwork/reader_resized.go

@@ -66,8 +66,12 @@ func resizedFromOriginal(ctx context.Context, a *artwork, artID model.ArtworkID,
 	return r, nil
 }
 
+// resizedKeyVersion invalidates resized entries cached by pre-blurhash versions: the refill is what
+// pulls the original through the tee, so warm entries would otherwise never backfill a hash.
+const resizedKeyVersion = "v1"
+
 func (a *resizedArtworkReader) Key() string {
-	baseKey := fmt.Sprintf("%s.%d", a.cacheKey, a.size)
+	baseKey := fmt.Sprintf("%s.%d.%s", a.cacheKey, a.size, resizedKeyVersion)
 	if a.square {
 		return baseKey + ".square"
 	}

core/artwork/reader_resized_test.go

@@ -6,12 +6,28 @@ import (
 	"errors"
 	"io"
 
+	"github.com/navidrome/navidrome/conf"
+	"github.com/navidrome/navidrome/conf/configtest"
 	"github.com/navidrome/navidrome/core/ffmpeg"
 	"github.com/navidrome/navidrome/tests"
 	. "github.com/onsi/ginkgo/v2"
 	. "github.com/onsi/gomega"
 )
 
+var _ = Describe("resizedArtworkReader.Key", func() {
+	BeforeEach(func() {
+		DeferCleanup(configtest.SetupConfig())
+		conf.Server.CoverArtQuality = 75
+	})
+
+	It("includes the cache version so pre-blurhash resized entries are invalidated", func() {
+		r := &resizedArtworkReader{cacheKey: "al-1.123", size: 300}
+		Expect(r.Key()).To(Equal("al-1.123.300.v1.75"))
+		r.square = true
+		Expect(r.Key()).To(Equal("al-1.123.300.v1.square"))
+	})
+})
+
 var _ = Describe("resizeImage", func() {
 	var mockFF *tests.MockFFmpeg
 	var r *resizedArtworkReader