feat(distribution): 站点归属兜底——未带站点编码时按访问域名识别分站

- 新增 middleware.SiteDomainFallback：显式头/query 优先，两者都为空时用 Request.Host / X-Forwarded-Host 反查 distribution_sites.domain，并回填 X-Distribution-Site-Code 供后续 sitecode.FromRequest 使用
- 新增 distribution.Repository.ResolveSiteCodeByDomain（仅取 status=active 的站点）
- 路由层在所有业务路由之前注册；域名到站点映射走 1 分钟进程内缓存（含负缓存）；查库异常不阻断请求、退化为按主站处理；回环/本地地址跳过查询
- 背景：站点归属此前完全依赖前端注入头，首屏探测失败或漏带就会把分站写入记成主站（2026-10-10 前已因此丢失一批分站注册归属）
- 新增单测：中间件（显式优先、域名解析、本地地址跳过、转发域兜底、缓存与容错）与仓储域名查询边界（大小写/空白/主站域名/停用站点）

backend/internal/modules/distribution/repository_test.go

@@ -42,6 +42,45 @@ func distributionTestDB(t *testing.T) *gorm.DB {
 	return db
 }
 
+// 站点归属兜底依赖「按访问域名反查站点编码」，这里覆盖大小写/空白、主站域名、停用站点等边界。
+func TestRepositoryResolveSiteCodeByDomain(t *testing.T) {
+	db := distributionTestDB(t)
+	repo := NewRepository(db)
+	if err := db.Create(&model.DistributionSite{
+		SiteCode: "gz_dist", SiteName: "天域商行", Domain: "tysh.hao858.com", Status: "active", TaxRateBPS: 100,
+	}).Error; err != nil {
+		t.Fatal(err)
+	}
+	if err := db.Create(&model.DistributionSite{
+		SiteCode: "down_site", SiteName: "停用站", Domain: "down.hao858.com", Status: "disabled", TaxRateBPS: 100,
+	}).Error; err != nil {
+		t.Fatal(err)
+	}
+
+	cases := []struct {
+		name   string
+		domain string
+		want   string
+	}{
+		{name: "命中并统一小写", domain: "TYSH.HAO858.COM", want: "gz_dist"},
+		{name: "去除首尾空白", domain: "  tysh.hao858.com  ", want: "gz_dist"},
+		{name: "主站域名无匹配", domain: "hao858.com", want: ""},
+		{name: "停用站点不解析", domain: "down.hao858.com", want: ""},
+		{name: "空域名", domain: "   ", want: ""},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := repo.ResolveSiteCodeByDomain(t.Context(), tc.domain)
+			if err != nil {
+				t.Fatalf("ResolveSiteCodeByDomain(%q) error = %v", tc.domain, err)
+			}
+			if got != tc.want {
+				t.Fatalf("ResolveSiteCodeByDomain(%q) = %q, want %q", tc.domain, got, tc.want)
+			}
+		})
+	}
+}
+
 func TestDistributionOrderSnapshotAndQueries(t *testing.T) {
 	db := distributionTestDB(t)
 	repo := NewRepository(db)