fix(安全): 修复上游校验与 URL 清理问题

增加请求阶段 DNS 解析校验，阻断重绑定到私网
补充默认透传 WWW-Authenticate 头，保留认证挑战
前端相对 URL 过滤拒绝 // 协议相对路径

测试: go test ./internal/repository -run TestGitHubReleaseServiceSuite
测试: go test ./internal/repository -run TestTurnstileServiceSuite
测试: go test ./internal/repository -run TestProxyProbeServiceSuite
测试: go test ./internal/repository -run TestClaudeUsageServiceSuite

backend/internal/repository/pricing_service.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/httpclient"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 )
@@ -16,9 +17,15 @@ type pricingRemoteClient struct {
 	httpClient *http.Client
 }
 
-func NewPricingRemoteClient() service.PricingRemoteClient {
+func NewPricingRemoteClient(cfg *config.Config) service.PricingRemoteClient {
+	allowPrivate := false
+	if cfg != nil {
+		allowPrivate = cfg.Security.URLAllowlist.AllowPrivateHosts
+	}
 	sharedClient, err := httpclient.GetClient(httpclient.Options{
-		Timeout: 30 * time.Second,
+		Timeout:            30 * time.Second,
+		ValidateResolvedIP: true,
+		AllowPrivateHosts:  allowPrivate,
 	})
 	if err != nil {
 		sharedClient = &http.Client{Timeout: 30 * time.Second}