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.go

@@ -3,8 +3,10 @@ package service
 import (
 	"context"
 	"strings"
+	"time"
 
 	"github.com/ShukeBta/MediaStationGo/internal/model"
+	"github.com/ShukeBta/MediaStationGo/internal/repository"
 )
 
 // ImageURL returns artwork for a media/series/season item id.
@@ -51,9 +53,97 @@ func (e *EmbyService) ImageURL(ctx context.Context, id, imageType string) (strin
 	} else if ok {
 		return pick(series.PosterURL, series.BackdropURL), nil
 	}
+	// id 既不是媒体也不是剧集组时,还可能是一个媒体库(Emby 客户端通过
+	// /Items/{libraryID}/Images/Primary 请求库封面)。复用网页的封面来源:
+	// cover_url 优先,否则挑库内评分最高的一张成员海报。
+	if raw := e.libraryCoverArtwork(ctx, id); raw != "" {
+		return raw, nil
+	}
 	return "", nil
 }
 
+// cachedLibraryCover returns a previously resolved library cover URL within TTL.
+func (e *EmbyService) cachedLibraryCover(id string) (string, bool) {
+	if e == nil || strings.TrimSpace(id) == "" {
+		return "", false
+	}
+	now := time.Now()
+	e.libraryCoverMu.Lock()
+	defer e.libraryCoverMu.Unlock()
+	entry, ok := e.libraryCoverCache[id]
+	if !ok || now.After(entry.expiresAt) {
+		if ok {
+			delete(e.libraryCoverCache, id)
+		}
+		return "", false
+	}
+	return entry.primary, true
+}
+
+func (e *EmbyService) rememberLibraryCover(id, cover string) {
+	if e == nil || strings.TrimSpace(id) == "" || strings.TrimSpace(cover) == "" {
+		return
+	}
+	e.libraryCoverMu.Lock()
+	defer e.libraryCoverMu.Unlock()
+	if e.libraryCoverCache == nil {
+		e.libraryCoverCache = make(map[string]embyArtworkCacheEntry)
+	}
+	if len(e.libraryCoverCache) > 4000 {
+		e.libraryCoverCache = make(map[string]embyArtworkCacheEntry)
+	}
+	e.libraryCoverCache[id] = embyArtworkCacheEntry{
+		primary:   cover,
+		expiresAt: time.Now().Add(embyVirtualCacheTTL),
+	}
+}
+
+// libraryCoverArtwork 解析媒体库封面,与网页媒体库封面逻辑一致:
+//  1. 用户手动设置的 cover_url 优先;
+//  2. 否则取库内最近的一批成员,按 seriesArtworkScore(同网页 artworkScore)
+//     挑出评分最高的那张海报作为封面。
+//
+// Emby 主图只能显示单张,因此返回的正是网页拼图里最靠前的那张。
+func (e *EmbyService) libraryCoverArtwork(ctx context.Context, id string) string {
+	if cached, ok := e.cachedLibraryCover(id); ok {
+		return cached
+	}
+	lib, err := e.repo.Library.FindByID(ctx, id)
+	if err != nil || lib == nil {
+		return ""
+	}
+	if strings.TrimSpace(lib.CoverURL) != "" {
+		cover := lib.CoverURL
+		e.rememberLibraryCover(id, cover)
+		return cover
+	}
+	var cover string
+	if rows, _, err := e.repo.Media.ListByLibraryFiltered(ctx, id, 0, 12, repository.MediaQueryFilter{IncludeNSFW: true}); err == nil {
+		var bestScore = -1
+		for i := range rows {
+			score := seriesArtworkScore(rows[i])
+			if score <= bestScore {
+				continue
+			}
+			bestScore = score
+			cover = rows[i].PosterURL
+			if cover == "" {
+				cover = rows[i].BackdropURL
+			}
+		}
+	}
+	if cover != "" {
+		e.rememberLibraryCover(id, cover)
+	}
+	return cover
+}
+
+// LibraryHasCover reports whether a library has an Emby-servable cover, so the
+// Views / virtual-folders payloads can advertise ImageTags.Primary to clients.
+func (e *EmbyService) LibraryHasCover(ctx context.Context, id string) bool {
+	return e.libraryCoverArtwork(ctx, id) != ""
+}
+
 func (e *EmbyService) mediaPrimaryArtwork(ctx context.Context, m *model.Media) string {
 	if m == nil {
 		return ""