feat(conversations): add notification mute functionality for conversations

- Rename Muted field to NotificationMuted across model, DTOs and converters
- Add UpdateNotificationMuted repository method with upsert support
- Add SetConversationNotificationMuted service method
- Add PUT /:id/notification_muted route for setting mute status
- Improve S3 storage resilience by skipping bucket check on access denied

internal/router/router.go

@@ -373,6 +373,7 @@ func (r *Router) setupRoutes() {
 			conversations.POST("/:id/messages", middleware.RequireVerified(r.userRepo), r.messageHandler.HandleSendMessage) // 发送消息
 			conversations.POST("/:id/read", r.messageHandler.HandleMarkRead)                                                // 标记已读
 			conversations.PUT("/:id/pinned", r.messageHandler.HandleSetConversationPinned)                                  // 置顶设置
+			conversations.PUT("/:id/notification_muted", r.messageHandler.HandleSetConversationNotificationMuted)          // 免打扰设置
 			conversations.GET("/unread/count", r.messageHandler.GetUnreadCount)                                             // 未读数
 			conversations.POST("/:id/typing", r.messageHandler.HandleTyping)                                                // 输入状态
 			conversations.DELETE("/:id/self", r.messageHandler.HandleDeleteConversationForSelf)                             // 删除会话(仅自己)