fix: 修复代码审核发现的10个问题（P0安全+P1数据一致性+P2性能优化）

P0: OpenAI SSE 错误消息 JSON 注入 — 使用 json.Marshal 替代 fmt.Sprintf
P1: subscription 续期包裹 Ent 事务确保原子性
P1: CSP nonce 生成处理 crypto/rand 错误，失败降级为 unsafe-inline
P1: singleflight 透传数据库真实错误，不再吞没为 not found
P1: GetUserSubscriptionsWithProgress 提取 calculateProgress 消除 N+1
P2: billing_cache/gateway_helper 迁移到 math/rand/v2 消除全局锁争用
P2: generateRandomID 降级分支增加原子计数器防碰撞
P2: CORS 非白名单 origin 不再设置 Allow-Headers/Methods/Max-Age
P2: Turnstile 验证移除 VerifyCode 空值跳过条件防绕过
P2: Redis Cluster Lua 脚本空 KEYS 添加兼容性警告注释

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/openai_gateway_handler.go

@@ -396,8 +396,19 @@ func (h *OpenAIGatewayHandler) handleStreamingAwareError(c *gin.Context, status
 		// Stream already started, send error as SSE event then close
 		flusher, ok := c.Writer.(http.Flusher)
 		if ok {
-			// Send error event in OpenAI SSE format
-			errorEvent := fmt.Sprintf(`event: error`+"\n"+`data: {"error": {"type": "%s", "message": "%s"}}`+"\n\n", errType, message)
+			// Send error event in OpenAI SSE format with proper JSON marshaling
+			errorData := map[string]any{
+				"error": map[string]string{
+					"type":    errType,
+					"message": message,
+				},
+			}
+			jsonBytes, err := json.Marshal(errorData)
+			if err != nil {
+				_ = c.Error(err)
+				return
+			}
+			errorEvent := fmt.Sprintf("event: error\ndata: %s\n\n", string(jsonBytes))
 			if _, err := fmt.Fprint(c.Writer, errorEvent); err != nil {
 				_ = c.Error(err)
 			}