Add durable backup retry queue scaffolding for TierStore

This is an exploratory commit to:

- Introduce BackupMode with BestEffortBackup and SemiSync semantics,
  plus a locally persisted backup retry queue for failed backup writes
  and removals.
- Add serialization for pending backup ops, deterministic
  queue encoding, durable enqueue semantics for SemiSync, and a retry
  task skeleton with backoff and stale-op detection.

Also update TierStore and queue docs to reflect the new backup-mode
model and at-least-once cleanup semantics.

Author: enigbe