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_snapshot_hydration_test.go

@@ -44,6 +44,10 @@ func (c *snapshotHydrationCache) TryLockBucket(ctx context.Context, bucket Sched
 	return true, nil
 }
 
+func (c *snapshotHydrationCache) UnlockBucket(ctx context.Context, bucket SchedulerBucket) error {
+	return nil
+}
+
 func (c *snapshotHydrationCache) ListBuckets(ctx context.Context) ([]SchedulerBucket, error) {
 	return nil, nil
 }