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/concurrency_service.go

@@ -2,12 +2,26 @@ package service
 
 import (
 	"context"
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
 	"log"
 	"time"
 
 	"sub2api/internal/service/ports"
 )
 
+// generateRequestID generates a unique request ID for concurrency slot tracking
+// Uses 8 random bytes (16 hex chars) for uniqueness
+func generateRequestID() string {
+	b := make([]byte, 8)
+	if _, err := rand.Read(b); err != nil {
+		// Fallback to nanosecond timestamp (extremely rare case)
+		return fmt.Sprintf("%x", time.Now().UnixNano())
+	}
+	return hex.EncodeToString(b)
+}
+
 const (
 	// Default extra wait slots beyond concurrency limit
 	defaultExtraWaitSlots = 20
@@ -41,7 +55,10 @@ func (s *ConcurrencyService) AcquireAccountSlot(ctx context.Context, accountID i
 		}, nil
 	}
 
-	acquired, err := s.cache.AcquireAccountSlot(ctx, accountID, maxConcurrency)
+	// Generate unique request ID for this slot
+	requestID := generateRequestID()
+
+	acquired, err := s.cache.AcquireAccountSlot(ctx, accountID, maxConcurrency, requestID)
 	if err != nil {
 		return nil, err
 	}
@@ -52,8 +69,8 @@ func (s *ConcurrencyService) AcquireAccountSlot(ctx context.Context, accountID i
 			ReleaseFunc: func() {
 				bgCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 				defer cancel()
-				if err := s.cache.ReleaseAccountSlot(bgCtx, accountID); err != nil {
-					log.Printf("Warning: failed to release account slot for %d: %v", accountID, err)
+				if err := s.cache.ReleaseAccountSlot(bgCtx, accountID, requestID); err != nil {
+					log.Printf("Warning: failed to release account slot for %d (req=%s): %v", accountID, requestID, err)
 				}
 			},
 		}, nil
@@ -77,7 +94,10 @@ func (s *ConcurrencyService) AcquireUserSlot(ctx context.Context, userID int64,
 		}, nil
 	}
 
-	acquired, err := s.cache.AcquireUserSlot(ctx, userID, maxConcurrency)
+	// Generate unique request ID for this slot
+	requestID := generateRequestID()
+
+	acquired, err := s.cache.AcquireUserSlot(ctx, userID, maxConcurrency, requestID)
 	if err != nil {
 		return nil, err
 	}
@@ -88,8 +108,8 @@ func (s *ConcurrencyService) AcquireUserSlot(ctx context.Context, userID int64,
 			ReleaseFunc: func() {
 				bgCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 				defer cancel()
-				if err := s.cache.ReleaseUserSlot(bgCtx, userID); err != nil {
-					log.Printf("Warning: failed to release user slot for %d: %v", userID, err)
+				if err := s.cache.ReleaseUserSlot(bgCtx, userID, requestID); err != nil {
+					log.Printf("Warning: failed to release user slot for %d (req=%s): %v", userID, requestID, err)
 				}
 			},
 		}, nil