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_test.go

@@ -6,6 +6,7 @@ import (
 	"net/http/httptest"
 	"testing"
 
+	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
 )
@@ -19,7 +20,13 @@ type PricingServiceSuite struct {
 
 func (s *PricingServiceSuite) SetupTest() {
 	s.ctx = context.Background()
-	client, ok := NewPricingRemoteClient().(*pricingRemoteClient)
+	client, ok := NewPricingRemoteClient(&config.Config{
+		Security: config.SecurityConfig{
+			URLAllowlist: config.URLAllowlistConfig{
+				AllowPrivateHosts: true,
+			},
+		},
+	}).(*pricingRemoteClient)
 	require.True(s.T(), ok, "type assertion failed")
 	s.client = client
 }