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

frontend/src/utils/url.ts

@@ -14,7 +14,7 @@ export function sanitizeUrl(value: string, options: SanitizeOptions = {}): strin
     return ''
   }
 
-  if (options.allowRelative && trimmed.startsWith('/')) {
+  if (options.allowRelative && trimmed.startsWith('/') && !trimmed.startsWith('//')) {
     return trimmed
   }