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_test.go

@@ -122,7 +122,10 @@ func TestSecurityHeaders(t *testing.T) {
 
 		csp := w.Header().Get("Content-Security-Policy")
 		assert.NotEmpty(t, csp)
-		assert.Equal(t, "default-src 'self'", csp)
+		// Policy is auto-enhanced with nonce and Cloudflare Insights domain
+		assert.Contains(t, csp, "default-src 'self'")
+		assert.Contains(t, csp, "'nonce-")
+		assert.Contains(t, csp, CloudflareInsightsDomain)
 	})
 
 	t.Run("csp_enabled_with_nonce_placeholder", func(t *testing.T) {
@@ -261,6 +264,83 @@ func TestNonceTemplate(t *testing.T) {
 	})
 }
 
+func TestEnhanceCSPPolicy(t *testing.T) {
+	t.Run("adds_nonce_placeholder_if_missing", func(t *testing.T) {
+		policy := "default-src 'self'; script-src 'self'"
+		enhanced := enhanceCSPPolicy(policy)
+
+		assert.Contains(t, enhanced, NonceTemplate)
+		assert.Contains(t, enhanced, CloudflareInsightsDomain)
+	})
+
+	t.Run("does_not_duplicate_nonce_placeholder", func(t *testing.T) {
+		policy := "default-src 'self'; script-src 'self' __CSP_NONCE__"
+		enhanced := enhanceCSPPolicy(policy)
+
+		// Should not duplicate
+		count := strings.Count(enhanced, NonceTemplate)
+		assert.Equal(t, 1, count)
+	})
+
+	t.Run("does_not_duplicate_cloudflare_domain", func(t *testing.T) {
+		policy := "default-src 'self'; script-src 'self' https://static.cloudflareinsights.com"
+		enhanced := enhanceCSPPolicy(policy)
+
+		count := strings.Count(enhanced, CloudflareInsightsDomain)
+		assert.Equal(t, 1, count)
+	})
+
+	t.Run("handles_policy_without_script_src", func(t *testing.T) {
+		policy := "default-src 'self'"
+		enhanced := enhanceCSPPolicy(policy)
+
+		assert.Contains(t, enhanced, "script-src")
+		assert.Contains(t, enhanced, NonceTemplate)
+		assert.Contains(t, enhanced, CloudflareInsightsDomain)
+	})
+
+	t.Run("preserves_existing_nonce", func(t *testing.T) {
+		policy := "script-src 'self' 'nonce-existing'"
+		enhanced := enhanceCSPPolicy(policy)
+
+		// Should not add placeholder if nonce already exists
+		assert.NotContains(t, enhanced, NonceTemplate)
+		assert.Contains(t, enhanced, "'nonce-existing'")
+	})
+}
+
+func TestAddToDirective(t *testing.T) {
+	t.Run("adds_to_existing_directive", func(t *testing.T) {
+		policy := "script-src 'self'; style-src 'self'"
+		result := addToDirective(policy, "script-src", "https://example.com")
+
+		assert.Contains(t, result, "script-src 'self' https://example.com")
+	})
+
+	t.Run("creates_directive_if_not_exists", func(t *testing.T) {
+		policy := "default-src 'self'"
+		result := addToDirective(policy, "script-src", "https://example.com")
+
+		assert.Contains(t, result, "script-src")
+		assert.Contains(t, result, "https://example.com")
+	})
+
+	t.Run("handles_directive_at_end_without_semicolon", func(t *testing.T) {
+		policy := "default-src 'self'; script-src 'self'"
+		result := addToDirective(policy, "script-src", "https://example.com")
+
+		assert.Contains(t, result, "https://example.com")
+	})
+
+	t.Run("handles_empty_policy", func(t *testing.T) {
+		policy := ""
+		result := addToDirective(policy, "script-src", "https://example.com")
+
+		assert.Contains(t, result, "script-src")
+		assert.Contains(t, result, "https://example.com")
+	})
+}
+
 // Benchmark tests
 func BenchmarkGenerateNonce(b *testing.B) {
 	for i := 0; i < b.N; i++ {