fix(安全): CSP 策略自动增强，无需配置文件修改即可生效

- 添加 enhanceCSPPolicy() 自动增强任何 CSP 策略
- 自动添加 nonce 占位符（如果策略中没有）
- 自动添加 Cloudflare Insights 域名
- 即使配置文件使用旧策略也能正常工作
- 添加 enhanceCSPPolicy 和 addToDirective 单元测试

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

backend/internal/server/middleware/security_headers.go

@@ -14,6 +14,8 @@ const (
 	CSPNonceKey = "csp_nonce"
 	// NonceTemplate is the placeholder in CSP policy for nonce
 	NonceTemplate = "__CSP_NONCE__"
+	// CloudflareInsightsDomain is the domain for Cloudflare Web Analytics
+	CloudflareInsightsDomain = "https://static.cloudflareinsights.com"
 )
 
 // GenerateNonce generates a cryptographically secure random nonce
@@ -40,6 +42,9 @@ func SecurityHeaders(cfg config.CSPConfig) gin.HandlerFunc {
 		policy = config.DefaultCSPPolicy
 	}
 
+	// Enhance policy with required directives (nonce placeholder and Cloudflare Insights)
+	policy = enhanceCSPPolicy(policy)
+
 	return func(c *gin.Context) {
 		c.Header("X-Content-Type-Options", "nosniff")
 		c.Header("X-Frame-Options", "DENY")
@@ -57,3 +62,55 @@ func SecurityHeaders(cfg config.CSPConfig) gin.HandlerFunc {
 		c.Next()
 	}
 }
+
+// enhanceCSPPolicy ensures the CSP policy includes nonce support and Cloudflare Insights domain.
+// This allows the application to work correctly even if the config file has an older CSP policy.
+func enhanceCSPPolicy(policy string) string {
+	// Add nonce placeholder to script-src if not present
+	if !strings.Contains(policy, NonceTemplate) && !strings.Contains(policy, "'nonce-") {
+		policy = addToDirective(policy, "script-src", NonceTemplate)
+	}
+
+	// Add Cloudflare Insights domain to script-src if not present
+	if !strings.Contains(policy, CloudflareInsightsDomain) {
+		policy = addToDirective(policy, "script-src", CloudflareInsightsDomain)
+	}
+
+	return policy
+}
+
+// addToDirective adds a value to a specific CSP directive.
+// If the directive doesn't exist, it will be added after default-src.
+func addToDirective(policy, directive, value string) string {
+	// Find the directive in the policy
+	directivePrefix := directive + " "
+	idx := strings.Index(policy, directivePrefix)
+
+	if idx == -1 {
+		// Directive not found, add it after default-src or at the beginning
+		defaultSrcIdx := strings.Index(policy, "default-src ")
+		if defaultSrcIdx != -1 {
+			// Find the end of default-src directive (next semicolon)
+			endIdx := strings.Index(policy[defaultSrcIdx:], ";")
+			if endIdx != -1 {
+				insertPos := defaultSrcIdx + endIdx + 1
+				// Insert new directive after default-src
+				return policy[:insertPos] + " " + directive + " 'self' " + value + ";" + policy[insertPos:]
+			}
+		}
+		// Fallback: prepend the directive
+		return directive + " 'self' " + value + "; " + policy
+	}
+
+	// Find the end of this directive (next semicolon or end of string)
+	endIdx := strings.Index(policy[idx:], ";")
+
+	if endIdx == -1 {
+		// No semicolon found, directive goes to end of string
+		return policy + " " + value
+	}
+
+	// Insert value before the semicolon
+	insertPos := idx + endIdx
+	return policy[:insertPos] + " " + value + policy[insertPos:]
+}