feat(distribution): 分销站点消息中心与钱包提现多租户独立分支隔离

- 新增 000070 数据库迁移：支持 chat_conversations、wallet_accounts、wallet_ledger、withdrawal_requests 按 distribution_site_id 隔离
- 消息中心：会话列表、未读数统计、站点专属客服及订单群聊与主站实现双向完全隔离
- 资金账户与提现：独立按 (user_id, distribution_site_id) 建账，分销订单自动结算至对应分站钱包，提现与流水相互独立
- 前台界面：Axios 自动透传 X-Distribution-Site-Code，钱包、提现与消息页面呈现分站专属标牌与独立说明

backend/internal/modules/chat/handler_user.go

@@ -21,7 +21,8 @@ func (h *Handler) UnreadCount(c *gin.Context) {
 		response.Unauthorized(c, "缺少用户上下文")
 		return
 	}
-	result, err := h.service.CountUnreadMessages(c.Request.Context(), Principal{Type: "user", ID: userID})
+	siteCode := parseSiteCode(c)
+	result, err := h.service.CountUnreadMessages(c.Request.Context(), Principal{Type: "user", ID: userID}, siteCode)
 	if err != nil {
 		writeChatError(c, err)
 		return
@@ -65,7 +66,11 @@ func (h *Handler) EnsureSupportConversation(c *gin.Context) {
 	var req EnsureSupportRequest
 	// scene 可选；空 body 仍按平台客服处理
 	_ = c.ShouldBindJSON(&req)
-	item, err := h.service.EnsureSupportConversation(c.Request.Context(), userID, req.Scene)
+	siteCode := parseSiteCode(c)
+	if siteCode == "" && req.DistributionSiteCode != "" {
+		siteCode = req.DistributionSiteCode
+	}
+	item, err := h.service.EnsureSupportConversation(c.Request.Context(), userID, req.Scene, siteCode)
 	if err != nil {
 		writeChatError(c, err)
 		return