fix(concurrency): 重构并发管理使用独立Key+原生TTL

问题：旧方案使用计数器模式，每次acquire都刷新TTL，导致僵尸数据永不过期

解决方案：
- 每个槽位使用独立Redis Key: concurrency:account:{id}:{requestID}
- 利用Redis原生TTL，每个槽位独立5分钟过期
- 服务崩溃后僵尸数据自动清理，无需手动干预
- 兼容多实例K8s部署

技术改动：
- 新增SCAN脚本统计活跃槽位数量
- 移除冗余的releaseScript，直接使用DEL命令
- Wait队列TTL只在首次创建时设置，避免刷新

backend/internal/service/ports/concurrency_cache.go

@@ -3,17 +3,21 @@ package ports
 import "context"
 
 // ConcurrencyCache defines cache operations for concurrency service
+// Uses independent keys per request slot with native Redis TTL for automatic cleanup
 type ConcurrencyCache interface {
-	// Slot management
-	AcquireAccountSlot(ctx context.Context, accountID int64, maxConcurrency int) (bool, error)
-	ReleaseAccountSlot(ctx context.Context, accountID int64) error
+	// Account slot management - each slot is a separate key with independent TTL
+	// Key format: concurrency:account:{accountID}:{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)
 
-	AcquireUserSlot(ctx context.Context, userID int64, maxConcurrency int) (bool, error)
-	ReleaseUserSlot(ctx context.Context, userID int64) error
+	// User slot management - each slot is a separate key with independent TTL
+	// Key format: concurrency:user:{userID}:{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
+	// Wait queue - uses counter with TTL set only on creation
 	IncrementWaitCount(ctx context.Context, userID int64, maxWait int) (bool, error)
 	DecrementWaitCount(ctx context.Context, userID int64) error
 }