feat(distribution): 站内信与通知体系多租户独立分支隔离

- 新增 000071 数据库迁移：notifications 表支持 distribution_site_id 并自动回填历史分销订单通知
- 通知自动归属：在事务内根据订单或申诉自动关联当前分销站点 ID
- 隔离查询与已读控制：列表、未读统计与批量已读均严格按站点隔离，防止主站与分站交叉泄漏
- 前台界面联动：消息中心通知 Tab 及移动端/PC端站内信页面展示分站专属标识与精准过滤

backend/internal/modules/notification/handler.go

@@ -40,7 +40,8 @@ func (h *Handler) List(c *gin.Context) {
 		return
 	}
 	page, pageSize := parsePagination(c)
-	result, err := h.service.List(c.Request.Context(), userID, page, pageSize)
+	siteCode := parseSiteCode(c)
+	result, err := h.service.List(c.Request.Context(), userID, page, pageSize, siteCode)
 	if err != nil {
 		writeNotificationError(c, err)
 		return
@@ -54,7 +55,8 @@ func (h *Handler) UnreadCount(c *gin.Context) {
 		response.Unauthorized(c, "缺少用户上下文")
 		return
 	}
-	result, err := h.service.UnreadCount(c.Request.Context(), userID)
+	siteCode := parseSiteCode(c)
+	result, err := h.service.UnreadCount(c.Request.Context(), userID, siteCode)
 	if err != nil {
 		writeNotificationError(c, err)
 		return
@@ -68,7 +70,8 @@ func (h *Handler) MarkAllRead(c *gin.Context) {
 		response.Unauthorized(c, "缺少用户上下文")
 		return
 	}
-	count, err := h.service.MarkAllRead(c.Request.Context(), userID)
+	siteCode := parseSiteCode(c)
+	count, err := h.service.MarkAllRead(c.Request.Context(), userID, siteCode)
 	if err != nil {
 		writeNotificationError(c, err)
 		return
@@ -87,13 +90,28 @@ func (h *Handler) MarkRead(c *gin.Context) {
 		response.BadRequest(c, "ID 不正确")
 		return
 	}
-	if err := h.service.MarkRead(c.Request.Context(), userID, id); err != nil {
+	siteCode := parseSiteCode(c)
+	if err := h.service.MarkRead(c.Request.Context(), userID, id, siteCode); err != nil {
 		writeNotificationError(c, err)
 		return
 	}
 	response.OK(c, gin.H{"read": true})
 }
 
+func parseSiteCode(c *gin.Context) string {
+	siteCode := c.Query("distribution_site_code")
+	if siteCode == "" {
+		siteCode = c.Query("site_code")
+	}
+	if siteCode == "" {
+		siteCode = c.GetHeader("X-Distribution-Site-Code")
+	}
+	if siteCode == "" {
+		siteCode = c.GetHeader("X-Site-Code")
+	}
+	return siteCode
+}
+
 func currentUserID(c *gin.Context) (uint64, bool) {
 	value, ok := c.Get(middleware.ContextUserID)
 	if !ok {