fix: use detached context for concurrency batch query to prevent all-zero display

The upstream v0.1.90 changed GetAccountConcurrencyBatch from individual
Lua script calls (which swallowed per-account errors) to a Redis pipeline
approach that propagates errors from rdb.Time() or pipe.Exec(). When the
HTTP request context is cancelled (e.g., browser abort), the entire batch
fails and the handler silently shows all concurrency as 0.

Fix: use context.WithTimeout(context.Background(), 3s) for the Redis call
so HTTP request cancellation doesn't affect the read-only concurrency query.

backend/internal/service/concurrency_service.go

@@ -331,8 +331,9 @@ func (s *ConcurrencyService) StartSlotCleanupWorker(accountRepo AccountRepositor
 	}()
 }
 
-// GetAccountConcurrencyBatch gets current concurrency counts for multiple accounts
-// Returns a map of accountID -> current concurrency count
+// GetAccountConcurrencyBatch gets current concurrency counts for multiple accounts.
+// Uses a detached context with timeout to prevent HTTP request cancellation from
+// causing the entire batch to fail (which would show all concurrency as 0).
 func (s *ConcurrencyService) GetAccountConcurrencyBatch(ctx context.Context, accountIDs []int64) (map[int64]int, error) {
 	if len(accountIDs) == 0 {
 		return map[int64]int{}, nil
@@ -344,5 +345,11 @@ func (s *ConcurrencyService) GetAccountConcurrencyBatch(ctx context.Context, acc
 		}
 		return result, nil
 	}
-	return s.cache.GetAccountConcurrencyBatch(ctx, accountIDs)
+
+	// Use a detached context so that a cancelled HTTP request doesn't cause
+	// the Redis pipeline to fail and return all-zero concurrency counts.
+	redisCtx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	return s.cache.GetAccountConcurrencyBatch(redisCtx, accountIDs)
 }