feat(pay): httpapi 加 CORS(web 跨域下单)+ 部署到 pangolin1 准备(#34/34A)

withCORS 中间件:仅白名单 origin(默认 pangolin.yanmeiai.com + pages.dev)返回 CORS 头、
OPTIONS 直接 204;New(svc, corsOrigins) + main 读 PAY_CORS_ORIGINS。为官网下单页跨域调 pay 铺路。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pay/internal/httpapi/cors.go

@@ -0,0 +1,40 @@
+package httpapi
+
+import (
+	"net/http"
+	"strings"
+)
+
+// withCORS lets the browser storefront (pangolin website) call the order API
+// cross-origin. Auth is server-to-server / bearer-less here (no cookies), so
+// Allow-Credentials is not needed. Only whitelisted origins get CORS headers;
+// OPTIONS preflight is answered directly.
+func withCORS(next http.Handler, allowed map[string]bool) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		origin := r.Header.Get("Origin")
+		if origin != "" && allowed[origin] {
+			h := w.Header()
+			h.Set("Access-Control-Allow-Origin", origin)
+			h.Add("Vary", "Origin")
+			h.Set("Access-Control-Allow-Methods", "GET, POST, OPTIONS")
+			h.Set("Access-Control-Allow-Headers", "Content-Type")
+			h.Set("Access-Control-Max-Age", "600")
+		}
+		if r.Method == http.MethodOptions {
+			w.WriteHeader(http.StatusNoContent)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}
+
+// parseOrigins builds the allowed-origin set from a comma-separated list.
+func parseOrigins(csv string) map[string]bool {
+	m := map[string]bool{}
+	for _, o := range strings.Split(csv, ",") {
+		if o = strings.TrimSpace(o); o != "" {
+			m[o] = true
+		}
+	}
+	return m
+}