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_common.go

@@ -1,14 +1,31 @@
 package chat
 
 import (
+	"strings"
+
 	"hfb_sys/backend/pkg/response"
 
 	"github.com/gin-gonic/gin"
 )
 
+func parseSiteCode(c *gin.Context) string {
+	for _, val := range []string{
+		c.Query("distribution_site_code"),
+		c.Query("site_code"),
+		c.GetHeader("X-Distribution-Site-Code"),
+		c.GetHeader("X-Site-Code"),
+	} {
+		if trimmed := strings.TrimSpace(val); trimmed != "" {
+			return trimmed
+		}
+	}
+	return ""
+}
+
 func (h *Handler) list(c *gin.Context, principal Principal) {
 	page, pageSize := parsePagination(c)
-	result, err := h.service.ListConversations(c.Request.Context(), principal, page, pageSize)
+	siteCode := parseSiteCode(c)
+	result, err := h.service.ListConversations(c.Request.Context(), principal, page, pageSize, siteCode)
 	if err != nil {
 		writeChatError(c, err)
 		return