refactor(sitecode): 统一各模块分销站点编码解析为公共包

- 新增 internal/sitecode 包：FromRequest 头优先解析、FromQueryThenHeader
  探测接口专用、ExtractHost/IsLocalHost 提取 Host 域名
- 替换 auth/order/listing/chat/notification/withdrawal/wallet/distribution
  八模块 17 处各自内联的解析逻辑，统一优先级与 TrimSpace 行为
- 修复 notification 漏 TrimSpace、探测接口会话头压过显式 query、
  GetPublicSiteInfo 把 localhost Host 当站点域名查库等不一致
- 顺手清理 auth 中无引用的 extractHost 死代码
- gofmt 修复 withdrawal/presenter、chat/file/payment 测试四个历史遗留格式问题

backend/internal/modules/notification/handler.go

@@ -5,6 +5,7 @@ import (
 	"strconv"
 
 	"hfb_sys/backend/internal/middleware"
+	"hfb_sys/backend/internal/sitecode"
 	"hfb_sys/backend/pkg/response"
 
 	"github.com/gin-gonic/gin"
@@ -99,17 +100,7 @@ func (h *Handler) MarkRead(c *gin.Context) {
 }
 
 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
+	return sitecode.FromRequest(c)
 }
 
 func currentUserID(c *gin.Context) (uint64, bool) {