refactor(model): give ItemImage an AspectRatio method

The zero/absent guards around width/height are ItemImage's own invariant, not
the Jellyfin adapter's. Moving them onto the model keeps one definition for
every consumer, so a second one cannot quietly disagree about what an unknown
ratio means.

model/artwork.go

@@ -27,6 +27,15 @@ type ItemImage struct {
 	ImageHeight int `structs:"-" json:"imageHeight,omitempty"`
 }
 
+// AspectRatio is the image's width/height, or nil when there is no image or its dimensions are
+// unknown (an unresolved item). Never guesses: a wrong ratio mis-shapes a client's placeholder.
+func (i ItemImage) AspectRatio() *float64 {
+	if i.ImageAbsent || i.ImageWidth <= 0 || i.ImageHeight <= 0 {
+		return nil
+	}
+	return new(float64(i.ImageWidth) / float64(i.ImageHeight))
+}
+
 // ItemArtwork is an entity's resolved artwork state. Hash=="" means known absent.
 type ItemArtwork struct {
 	ItemKind  string `structs:"item_kind"`

model/artwork_test.go

@@ -52,6 +52,24 @@ var _ = Describe("ItemImage JSON", func() {
 		Expect(out).ToNot(HaveKey("imageHeight"))
 	})
 
+	Describe("AspectRatio", func() {
+		It("returns width/height", func() {
+			img := model.ItemImage{ImageHash: "abc", ImageWidth: 1200, ImageHeight: 800}
+			Expect(*img.AspectRatio()).To(BeNumerically("~", 1.5, 0.0001))
+		})
+
+		It("returns nil when a dimension is missing, so callers never guess a ratio", func() {
+			Expect(model.ItemImage{ImageHash: "abc"}.AspectRatio()).To(BeNil())
+			Expect(model.ItemImage{ImageHash: "abc", ImageWidth: 1200}.AspectRatio()).To(BeNil())
+			Expect(model.ItemImage{ImageHash: "abc", ImageHeight: 800}.AspectRatio()).To(BeNil())
+		})
+
+		It("returns nil for a known-absent image, whatever the dimensions say", func() {
+			img := model.ItemImage{ImageAbsent: true, ImageWidth: 1200, ImageHeight: 800}
+			Expect(img.AspectRatio()).To(BeNil())
+		})
+	})
+
 	It("exposes known-absent artwork so clients can skip the request", func() {
 		ar := model.Artist{ID: "ar-1", Name: "Artist"}
 		ar.ImageAbsent = true

server/jellyfin/dto/mappers.go

@@ -232,9 +232,8 @@ func primaryImage(img model.ItemImage, fallback string, fields Fields) (tag stri
 	if img.BlurHash != "" {
 		blurs = map[string]map[string]string{"Primary": {tag: img.BlurHash}}
 	}
-	// Dimensions are unknown while an item is unresolved; omit rather than guess a ratio.
-	if fields.Has("PrimaryImageAspectRatio") && img.ImageWidth > 0 && img.ImageHeight > 0 {
-		ratio = new(float64(img.ImageWidth) / float64(img.ImageHeight))
+	if fields.Has("PrimaryImageAspectRatio") {
+		ratio = img.AspectRatio()
 	}
 	return tag, blurs, ratio
 }