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/proxy_probe_service_test.go

@@ -20,7 +20,10 @@ type ProxyProbeServiceSuite struct {
 
 func (s *ProxyProbeServiceSuite) SetupTest() {
 	s.ctx = context.Background()
-	s.prober = &proxyProbeService{ipInfoURL: "http://ipinfo.test/json"}
+	s.prober = &proxyProbeService{
+		ipInfoURL:         "http://ipinfo.test/json",
+		allowPrivateHosts: true,
+	}
 }
 
 func (s *ProxyProbeServiceSuite) TearDownTest() {