feat(安全): 实现 CSP nonce 支持解决内联脚本安全问题

- 添加 GenerateNonce() 生成加密安全的随机 nonce
- SecurityHeaders 中间件为每个请求生成唯一 nonce
- CSP 策略支持 __CSP_NONCE__ 占位符动态替换
- embed_on.go 注入的内联脚本添加 nonce 属性
- 添加 Cloudflare Insights 域名到 CSP 允许列表
- 添加完整单元测试，覆盖率达到 89.8%

解决的问题：
- 内联脚本违反 CSP script-src 指令
- Cloudflare Insights beacon.min.js 加载被阻止

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/config/config.go

@@ -19,7 +19,9 @@ const (
 	RunModeSimple   = "simple"
 )
 
-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'"
+// DefaultCSPPolicy is the default Content-Security-Policy with nonce support
+// __CSP_NONCE__ will be replaced with actual nonce at request time by the SecurityHeaders middleware
+const DefaultCSPPolicy = "default-src 'self'; script-src 'self' __CSP_NONCE__ https://challenges.cloudflare.com https://static.cloudflareinsights.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