Add gated cover byte fetcher

backend/internal/store/store_test.go

@@ -1352,3 +1352,25 @@ func TestKaganeCoverIsContentAddressedOnFilesystem(t *testing.T) {
 		t.Fatalf("covers still has body column")
 	}
 }
+
+func TestCoverStoreAcceptsAnySourceURL(t *testing.T) {
+	s := newTestStore(t)
+	const sourceURL = "https://cdn.example/covers/series.jpg"
+	want := []byte("cover-bytes")
+	if err := s.PutCover(sourceURL, want, "image/jpeg"); err != nil {
+		t.Fatalf("PutCover: %v", err)
+	}
+	got, contentType, ok, err := s.GetCover(sourceURL)
+	if err != nil {
+		t.Fatalf("GetCover: %v", err)
+	}
+	if !ok || !bytes.Equal(got, want) || contentType != "image/jpeg" {
+		t.Fatalf("GetCover = (%q, %q, %v), want (%q, image/jpeg, true)", got, contentType, ok, want)
+	}
+	if err := s.PutCover("https://cdn.example/not-image", []byte("html"), "text/html"); err == nil {
+		t.Fatal("PutCover accepted a non-image")
+	}
+	if _, _, ok, err := s.GetCover("https://cdn.example/not-image"); err != nil || ok {
+		t.Fatalf("rejected cover = found %v, err %v; want missing", ok, err)
+	}
+}