fix: 修复 CSP 策略阻止 Cloudflare Turnstile 加载的问题

在 script-src 和 frame-src 中添加 challenges.cloudflare.com 域名，
允许 Turnstile 脚本加载和 iframe 渲染。

backend/internal/config/config.go

@@ -17,7 +17,7 @@ const (
 	RunModeSimple   = "simple"
 )
 
-const DefaultCSPPolicy = "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; img-src 'self' data: https:; font-src 'self' data: https://fonts.gstatic.com; connect-src 'self' https:; frame-ancestors 'none'; base-uri 'self'; form-action 'self'"
+const DefaultCSPPolicy = "default-src 'self'; script-src 'self' https://challenges.cloudflare.com; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; img-src 'self' data: https:; font-src 'self' data: https://fonts.gstatic.com; connect-src 'self' https:; frame-src https://challenges.cloudflare.com; frame-ancestors 'none'; base-uri 'self'; form-action 'self'"
 
 // 连接池隔离策略常量
 // 用于控制上游 HTTP 连接池的隔离粒度，影响连接复用和资源消耗

deploy/config.example.yaml

@@ -97,7 +97,7 @@ security:
     enabled: true
     # Default CSP policy (override if you host assets on other domains)
     # 默认 CSP 策略（如果静态资源托管在其他域名，请自行覆盖）
-    policy: "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; img-src 'self' data: https:; font-src 'self' data: https://fonts.gstatic.com; connect-src 'self' https:; frame-ancestors 'none'; base-uri 'self'; form-action 'self'"
+    policy: "default-src 'self'; script-src 'self' https://challenges.cloudflare.com; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; img-src 'self' data: https:; font-src 'self' data: https://fonts.gstatic.com; connect-src 'self' https:; frame-src https://challenges.cloudflare.com; frame-ancestors 'none'; base-uri 'self'; form-action 'self'"
   proxy_probe:
     # Allow skipping TLS verification for proxy probe (debug only)
     # 允许代理探测时跳过 TLS 证书验证（仅用于调试）