feat: 交付官方 Docker 运行时，外观改背景图并下线自定义 CSS/JS

站点/后台分轨背景与用户列表排序一并落地；生产 CORS 改走 SITE_URL，健康检查带版本号。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/service/upload.go

@@ -29,6 +29,10 @@ const (
 	// 帖子插图：允许 JPEG/PNG/WebP，不强制转码
 	ImageMaxBytes = 5 << 20 // 5 MiB
 	ImageMaxDim   = 4096
+
+	// 站点背景图（前台/后台墙纸）
+	BackgroundMaxBytes = 8 << 20 // 8 MiB
+	BackgroundMaxDim   = 8192
 )
 
 // UploadService 附件上传：落盘到 data/uploads，元信息入库 attachments
@@ -64,7 +68,10 @@ func (s *UploadService) EnsureDir() error {
 	if err := os.MkdirAll(filepath.Join(s.dir, "avatars"), 0o755); err != nil {
 		return err
 	}
-	return os.MkdirAll(filepath.Join(s.dir, "images"), 0o755)
+	if err := os.MkdirAll(filepath.Join(s.dir, "images"), 0o755); err != nil {
+		return err
+	}
+	return os.MkdirAll(filepath.Join(s.dir, "backgrounds"), 0o755)
 }
 
 // SaveAvatar 保存裁剪后的 WebP 头像：校验魔数/大小/尺寸 → 落盘 → 写附件记录 → 更新用户头像
@@ -267,6 +274,119 @@ func (s *UploadService) SaveImage(userID uint, src io.Reader) (*model.Attachment
 	return att, nil
 }
 
+// SaveBackground 保存站点背景图到 uploads/backgrounds，不进用户媒体库
+func (s *UploadService) SaveBackground(src io.Reader) (string, error) {
+	if src == nil {
+		return "", errors.New("文件为空")
+	}
+	tooLarge := func() error {
+		return errors.New("背景图不能超过 8MB")
+	}
+
+	head := make([]byte, 12)
+	n, err := io.ReadFull(src, head)
+	if err != nil && !errors.Is(err, io.ErrUnexpectedEOF) && !errors.Is(err, io.EOF) {
+		return "", errors.New("读取图片失败")
+	}
+	if n == 0 {
+		return "", errors.New("文件为空")
+	}
+	format, err := detectImageFormat(head[:n])
+	if err != nil {
+		return "", err
+	}
+
+	if err := os.MkdirAll(filepath.Join(s.dir, "backgrounds"), 0o755); err != nil {
+		return "", err
+	}
+
+	nameBytes := make([]byte, 16)
+	if _, err := rand.Read(nameBytes); err != nil {
+		return "", err
+	}
+	filename := hex.EncodeToString(nameBytes) + format.ext
+	fullPath := filepath.Join(s.dir, "backgrounds", filename)
+	tmp := fullPath + ".partial"
+
+	f, err := os.OpenFile(tmp, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o644)
+	if err != nil {
+		return "", err
+	}
+	remain := BackgroundMaxBytes + 1 - int64(n)
+	if remain < 0 {
+		_ = f.Close()
+		_ = os.Remove(tmp)
+		return "", tooLarge()
+	}
+	if _, err := f.Write(head[:n]); err != nil {
+		_ = f.Close()
+		_ = os.Remove(tmp)
+		return "", err
+	}
+	writtenRest, copyErr := io.Copy(f, io.LimitReader(src, remain))
+	_ = f.Close()
+	if copyErr != nil {
+		_ = os.Remove(tmp)
+		return "", copyErr
+	}
+	total := int64(n) + writtenRest
+	if total > BackgroundMaxBytes {
+		_ = os.Remove(tmp)
+		return "", tooLarge()
+	}
+
+	rf, err := os.Open(tmp)
+	if err != nil {
+		_ = os.Remove(tmp)
+		return "", err
+	}
+	w, h, err := decodeImageSizeReader(rf, format.mime)
+	_ = rf.Close()
+	if err != nil {
+		_ = os.Remove(tmp)
+		return "", err
+	}
+	if w < 1 || h < 1 {
+		_ = os.Remove(tmp)
+		return "", errors.New("无效的图片尺寸")
+	}
+	if w > BackgroundMaxDim || h > BackgroundMaxDim {
+		_ = os.Remove(tmp)
+		return "", errors.New("背景图边长不能超过 8192px")
+	}
+	if err := os.Rename(tmp, fullPath); err != nil {
+		_ = os.Remove(tmp)
+		return "", err
+	}
+	return "/uploads/backgrounds/" + filename, nil
+}
+
+// BackgroundFileExists 确认 URL 对应文件在 backgrounds 目录内
+func (s *UploadService) BackgroundFileExists(url string) bool {
+	rel := BgRelPath(url)
+	if rel == "" {
+		return false
+	}
+	abs := filepath.Join(s.dir, rel)
+	info, err := os.Stat(abs)
+	return err == nil && !info.IsDir()
+}
+
+// RemoveBackgroundIfUnused 旧墙纸不再被前台或后台引用时删除物理文件
+func (s *UploadService) RemoveBackgroundIfUnused(oldURL, siteURL, adminURL string) {
+	if oldURL == "" || oldURL == siteURL || oldURL == adminURL {
+		return
+	}
+	rel := BgRelPath(oldURL)
+	if rel == "" {
+		return
+	}
+	abs := filepath.Join(s.dir, rel)
+	if err := os.Remove(abs); err != nil && !os.IsNotExist(err) {
+		log.Printf("[upload] 删除背景图失败 path=%s: %v", abs, err)
+	}
+}
+
 // UseAvatar 选用一张【本人历史上传】的头像
 func (s *UploadService) UseAvatar(userID uint, url string) error {
 	url = strings.TrimSpace(url)