perf(后端): 完成性能优化与连接池配置

新增 DB/Redis 连接池配置与校验，并补充单测

网关请求体大小限制与 413 处理

HTTP/req 客户端池化并调整上游连接池默认值

并发槽位改为 ZSET+Lua 与指数退避

用量统计改 SQL 聚合并新增索引迁移

计费缓存写入改工作池并补测试/基准

测试: 在 backend/ 下运行 go test ./...

backend/internal/service/concurrency_service.go

@@ -9,22 +9,22 @@ import (
 	"time"
 )
 
-// ConcurrencyCache defines cache operations for concurrency service
-// Uses independent keys per request slot with native Redis TTL for automatic cleanup
+// ConcurrencyCache 定义并发控制的缓存接口
+// 使用有序集合存储槽位，按时间戳清理过期条目
 type ConcurrencyCache interface {
-	// Account slot management - each slot is a separate key with independent TTL
-	// Key format: concurrency:account:{accountID}:{requestID}
+	// 账号槽位管理
+	// 键格式: concurrency:account:{accountID}（有序集合，成员为 requestID）
 	AcquireAccountSlot(ctx context.Context, accountID int64, maxConcurrency int, requestID string) (bool, error)
 	ReleaseAccountSlot(ctx context.Context, accountID int64, requestID string) error
 	GetAccountConcurrency(ctx context.Context, accountID int64) (int, error)
 
-	// User slot management - each slot is a separate key with independent TTL
-	// Key format: concurrency:user:{userID}:{requestID}
+	// 用户槽位管理
+	// 键格式: concurrency:user:{userID}（有序集合，成员为 requestID）
 	AcquireUserSlot(ctx context.Context, userID int64, maxConcurrency int, requestID string) (bool, error)
 	ReleaseUserSlot(ctx context.Context, userID int64, requestID string) error
 	GetUserConcurrency(ctx context.Context, userID int64) (int, error)
 
-	// Wait queue - uses counter with TTL set only on creation
+	// 等待队列计数（只在首次创建时设置 TTL）
 	IncrementWaitCount(ctx context.Context, userID int64, maxWait int) (bool, error)
 	DecrementWaitCount(ctx context.Context, userID int64) error
 }