feat: crash recovery and graceful shutdown (story 1.9)

[vieiralucas]

Summary

• Implement crash recovery that scans for expired leases on startup and reclaims them so messages re-enter the ready pool
• Add WAL flush on graceful shutdown to ensure all writes are durable before exit
• Add parse_lease_expiry_key() for extracting queue_id and msg_id from expiry keys
• Add flush() method to Storage trait, implemented via flush_wal(true) in RocksDB

Test plan

• recovery_preserves_messages_after_restart — enqueue 5 messages, restart scheduler, verify all delivered
• recovery_reclaims_expired_leases — create expired lease, restart, verify reclaimed and message delivered to consumer
• recovery_preserves_queue_definitions — create 3 queues, restart, verify all present
• shutdown_flushes_wal — enqueue, shutdown, reopen storage from disk, verify data survived
• All 48 tests pass, clippy clean, fmt clean

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Summary by cubic

Adds crash recovery and graceful shutdown: on startup we reclaim expired leases so messages re-enter the ready pool; on shutdown we flush the RocksDB WAL for durability. Completes Story 1.9 and closes Epic 1.

• New Features
  • Run recovery before the main loop; log reclaimed count and queue summary.
  • Scan lease_expiry <= now; atomically delete lease and expiry.
  • Storage::flush added; RocksDB uses flush_wal(true) on shutdown.
  • Added parse_lease_expiry_key() to extract queue_id and msg_id.

^{Written for commit ac33d31. Summary will update on new commits.}

[cubic-dev-ai]

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