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.
