fix(scheduler): resolve SetSnapshot race conditions and remove usage throttle

Backend: Fix three race conditions in SetSnapshot that caused account
scheduling anomalies and broken sticky sessions:
- Use Lua CAS script for atomic version activation, preventing version
  rollback when concurrent goroutines write snapshots simultaneously
- Add UnlockBucket to release rebuild lock immediately after completion
  instead of waiting 30s TTL expiry
- Replace immediate DEL of old snapshots with 60s EXPIRE grace period,
  preventing readers from hitting empty ZRANGE during version switches

Frontend: Remove serial queue throttle (1-2s delay per request) from
usage loading since backend now uses passive sampling. All usage
requests execute immediately in parallel.

backend/internal/service/scheduler_cache.go

@@ -59,6 +59,8 @@ type SchedulerCache interface {
 	UpdateLastUsed(ctx context.Context, updates map[int64]time.Time) error
 	// TryLockBucket 尝试获取分桶重建锁。
 	TryLockBucket(ctx context.Context, bucket SchedulerBucket, ttl time.Duration) (bool, error)
+	// UnlockBucket 释放分桶重建锁。
+	UnlockBucket(ctx context.Context, bucket SchedulerBucket) error
 	// ListBuckets 返回已注册的分桶集合。
 	ListBuckets(ctx context.Context) ([]SchedulerBucket, error)
 	// GetOutboxWatermark 读取 outbox 水位。