fix(api): lazy cover self-heal + tmp-file skip

backend/internal/api/content.go

@@ -36,8 +36,42 @@ func (a *api) serveCover(c *gin.Context) {
 	}
 	a.immutable(c)
 	dir := bookfile.CoverDir(a.cfg.CacheDir, bookfile.DirKey(b.ID, bookfile.Hash(b.FileSize, b.ModTS)))
-	if entries, e := os.ReadDir(dir); e == nil && len(entries) > 0 {
-		http.ServeFile(c.Writer, c.Request, filepath.Join(dir, entries[0].Name()))
+	if entries, e := os.ReadDir(dir); e == nil {
+		for _, en := range entries { // 跳过写一半的 .tmp 落盘中间态
+			if !strings.Contains(en.Name(), ".tmp") {
+				http.ServeFile(c.Writer, c.Request, filepath.Join(dir, en.Name()))
+				return
+			}
+		}
+	}
+	if b.Format == "cbz" || b.Format == "epub" { // 自愈:缓存丢了就地抽封面(重启/卷漂移/扫描器还没跑到)
+		if root, ok := a.bookRoot(c, b); ok {
+			if f, size, ok := a.openBook(c, b, root); ok {
+				defer f.Close()
+				var img []byte
+				var ext string
+				var e error
+				if b.Format == "cbz" {
+					img, ext, e = bookfile.CBZCover(f, size)
+				} else {
+					img, ext, e = bookfile.EPUBCover(f, size)
+				}
+				if e == nil {
+					dst := filepath.Join(dir, "cover"+ext)
+					if e := os.MkdirAll(dir, 0o755); e == nil {
+						tmp := fmt.Sprintf("%s.tmp-%d", dst, time.Now().UnixNano()) // 并发幂等:唯一 tmp + rename 原子
+						if e := os.WriteFile(tmp, img, 0o644); e == nil {
+							if e := os.Rename(tmp, dst); e == nil {
+								http.ServeFile(c.Writer, c.Request, dst)
+							}
+						}
+						os.Remove(tmp)
+					}
+				}
+			}
+		}
+	}
+	if c.Writer.Written() { // openBook/bookRoot 已写 403/404/500,不再叠加占位图
 		return
 	}
 	c.Data(http.StatusOK, "image/svg+xml", []byte(defaultCover))