优化客户端封面获取逻辑

修改emby客户端获取封面逻辑，修复没有设置封面导致封面为空的问题

internal/service/emby_artwork_test.go

@@ -0,0 +1,108 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/ShukeBta/MediaStationGo/internal/model"
+)
+
+func TestImageURLFallsBackToLibraryCoverURL(t *testing.T) {
+	svc := newTestEmbyService(t)
+	lib := model.Library{Name: "电影", Path: "/media/movies", Type: "movie", Enabled: true, CoverURL: "https://example.com/lib-cover.jpg"}
+	if err := svc.repo.Library.Create(t.Context(), &lib); err != nil {
+		t.Fatalf("create library: %v", err)
+	}
+
+	raw, err := svc.ImageURL(t.Context(), lib.ID, "Primary")
+	if err != nil {
+		t.Fatalf("ImageURL: %v", err)
+	}
+	if raw != lib.CoverURL {
+		t.Fatalf("ImageURL for library = %q, want cover_url %q", raw, lib.CoverURL)
+	}
+}
+
+func TestImageURLFallsBackToBestMemberPoster(t *testing.T) {
+	svc := newTestEmbyService(t)
+	lib := model.Library{Name: "电影", Path: "/media/movies", Type: "movie", Enabled: true} // no cover_url
+	if err := svc.repo.Library.Create(t.Context(), &lib); err != nil {
+		t.Fatalf("create library: %v", err)
+	}
+	members := []model.Media{
+		{LibraryID: lib.ID, Title: "A", Path: "/media/movies/a.mkv", PosterURL: "https://example.com/a/backdrop.jpg"},
+		{LibraryID: lib.ID, Title: "B", Path: "/media/movies/b.mkv", PosterURL: "https://example.com/b/poster.jpg"},
+		{LibraryID: lib.ID, Title: "C", Path: "/media/movies/c.mkv", PosterURL: "https://example.com/c/actor.jpg"},
+	}
+	for i := range members {
+		if err := svc.repo.DB.Create(&members[i]).Error; err != nil {
+			t.Fatalf("create media: %v", err)
+		}
+	}
+
+	raw, err := svc.ImageURL(t.Context(), lib.ID, "Primary")
+	if err != nil {
+		t.Fatalf("ImageURL: %v", err)
+	}
+	// 最高分是 poster.jpg(score 40),其次是默认海报(30),actor(10)。
+	if raw != "https://example.com/b/poster.jpg" {
+		t.Fatalf("ImageURL for library = %q, want best member poster", raw)
+	}
+}
+
+func TestImageURLReturnsEmptyForUnknownID(t *testing.T) {
+	svc := newTestEmbyService(t)
+	raw, err := svc.ImageURL(t.Context(), "not-a-library-or-media", "Primary")
+	if err != nil {
+		t.Fatalf("ImageURL: %v", err)
+	}
+	if raw != "" {
+		t.Fatalf("ImageURL for unknown id = %q, want empty", raw)
+	}
+}
+
+func TestViewsAdvertisesPrimaryImageWhenLibraryHasCover(t *testing.T) {
+	svc := newTestEmbyService(t)
+	withCover := model.Library{Name: "封库", Path: "/media/set", Type: "movie", Enabled: true}
+	if err := svc.repo.Library.Create(t.Context(), &withCover); err != nil {
+		t.Fatalf("create library: %v", err)
+	}
+	if err := svc.repo.DB.Create(&model.Media{
+		Base:      model.Base{ID: "m1"},
+		LibraryID: withCover.ID,
+		Title:     "M1",
+		Path:      "/media/set/m1.mkv",
+		PosterURL: "https://example.com/m1/poster.jpg",
+	}).Error; err != nil {
+		t.Fatalf("create media: %v", err)
+	}
+	noCover := model.Library{Name: "空库", Path: "/media/empty", Type: "movie", Enabled: true}
+	if err := svc.repo.Library.Create(t.Context(), &noCover); err != nil {
+		t.Fatalf("create library: %v", err)
+	}
+
+	views, err := svc.Views(t.Context(), "user-1")
+	if err != nil {
+		t.Fatalf("Views: %v", err)
+	}
+	items := views["Items"].([]map[string]any)
+	found := map[string]map[string]any{}
+	for _, it := range items {
+		found[it["Id"].(string)] = it
+	}
+
+	withImageTags, ok := found[withCover.ID]["ImageTags"].(map[string]string)
+	if !ok {
+		t.Fatalf("library with cover should have ImageTags, got %#v", found[withCover.ID]["ImageTags"])
+	}
+	if withImageTags["Primary"] != withCover.ID {
+		t.Fatalf("library with cover should advertise ImageTags.Primary, got %#v", withImageTags)
+	}
+
+	emptyTags, ok := found[noCover.ID]["ImageTags"].(map[string]string)
+	if !ok {
+		t.Fatalf("library without cover should have empty ImageTags, got %#v", found[noCover.ID]["ImageTags"])
+	}
+	if _, has := emptyTags["Primary"]; has {
+		t.Fatalf("library without cover should NOT advertise Primary, got %#v", emptyTags)
+	}
+}