feat: 后台增加开放消息开关

关闭后前台隐藏私聊/群聊入口并由后端拦截；同步去掉公告页重复计数胶囊。

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

backend/handler/chat.go

@@ -47,15 +47,33 @@ func (h *Handlers) chatOversee(userID uint) bool {
 	return service.CanOverseeChat(actor.Role)
 }
 
+// RequireMessagesOpen 前台群聊/私聊闸门；关闭后一律 403。管理端 /api/admin/chat 不受影响。
+func (h *Handlers) RequireMessagesOpen(c *gin.Context) {
+	if h.Setting == nil {
+		c.AbortWithStatusJSON(http.StatusInternalServerError, gin.H{"error": "消息暂时不可用"})
+		return
+	}
+	allow, err := h.Setting.AllowMessages()
+	if err != nil {
+		c.AbortWithStatusJSON(http.StatusInternalServerError, gin.H{"error": "消息暂时不可用"})
+		return
+	}
+	if !allow {
+		c.AbortWithStatusJSON(http.StatusForbidden, gin.H{"error": "本站不支持消息"})
+		return
+	}
+	c.Next()
+}
+
 // CreateChatRoom 建群（可公开/私密，私密可带初始成员）
 func (h *Handlers) CreateChatRoom(c *gin.Context) {
 	claims := middleware.CurrentUser(c)
 	var req struct {
-		Name             string   `json:"name"`
-		Description      string   `json:"description"`
-		IsPrivate        bool     `json:"is_private"`
-		MemberIDs        []uint   `json:"member_ids"`
-		MemberUsernames  []string `json:"member_usernames"`
+		Name            string   `json:"name"`
+		Description     string   `json:"description"`
+		IsPrivate       bool     `json:"is_private"`
+		MemberIDs       []uint   `json:"member_ids"`
+		MemberUsernames []string `json:"member_usernames"`
 	}
 	if err := c.ShouldBindJSON(&req); err != nil {
 		c.JSON(http.StatusBadRequest, gin.H{"error": "请求参数无效"})