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/handler/gateway_handler_warmup_intercept_unit_test.go

@@ -50,6 +50,9 @@ func (f *fakeSchedulerCache) UpdateLastUsed(_ context.Context, _ map[int64]time.
 func (f *fakeSchedulerCache) TryLockBucket(_ context.Context, _ service.SchedulerBucket, _ time.Duration) (bool, error) {
 	return true, nil
 }
+func (f *fakeSchedulerCache) UnlockBucket(_ context.Context, _ service.SchedulerBucket) error {
+	return nil
+}
 func (f *fakeSchedulerCache) ListBuckets(_ context.Context) ([]service.SchedulerBucket, error) {
 	return nil, nil
 }