fix(backend): single error body on pages paths, reject newline entry names, empty-index cache skip

backend/internal/api/content.go

@@ -36,7 +36,7 @@ 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, err := os.ReadDir(dir); err == nil && len(entries) > 0 {
+	if entries, e := os.ReadDir(dir); e == nil && len(entries) > 0 {
 		http.ServeFile(c.Writer, c.Request, filepath.Join(dir, entries[0].Name()))
 		return
 	}
@@ -85,7 +85,7 @@ func (a *api) openBook(c *gin.Context, b store.Book, root string) (*os.File, int
 func (a *api) pageIndex(c *gin.Context, b store.Book, root string) ([]string, error) {
 	hash := bookfile.Hash(b.FileSize, b.ModTS)
 	key := fmt.Sprintf("pagesidx:%d:%s", b.ID, hash)
-	if v, ok := a.rdb.Get(c, key); ok {
+	if v, ok := a.rdb.Get(c, key); ok && v != "" {
 		return strings.Split(v, "\n"), nil
 	}
 	f, size, ok := a.openBook(c, b, root)
@@ -97,7 +97,9 @@ func (a *api) pageIndex(c *gin.Context, b store.Book, root string) ([]string, er
 	if e != nil {
 		return nil, e
 	}
-	a.rdb.Set(c, key, strings.Join(idx, "\n"), 7*24*time.Hour)
+	if len(idx) > 0 { // 空索引不缓存,否则 warm 命中 "" 会 Split 出幽灵页
+		a.rdb.Set(c, key, strings.Join(idx, "\n"), 7*24*time.Hour)
+	}
 	return idx, nil
 }
 
@@ -116,6 +118,9 @@ func (a *api) pagesCount(c *gin.Context) {
 	}
 	idx, e := a.pageIndex(c, b, root)
 	if e != nil {
+		if c.Writer.Written() {
+			return // openBook 已写 403/404,不再叠加 422
+		}
 		err(c, http.StatusUnprocessableEntity, "broken", e.Error())
 		return
 	}
@@ -142,6 +147,9 @@ func (a *api) page(c *gin.Context) {
 	}
 	idx, e := a.pageIndex(c, b, root)
 	if e != nil {
+		if c.Writer.Written() {
+			return // openBook 已写 403/404,不再叠加 422
+		}
 		err(c, http.StatusUnprocessableEntity, "broken", e.Error())
 		return
 	}