feat: message ack with atomic cleanup (story 1.8)

[vieiralucas]

Summary

• Implement handle_ack in scheduler: validates lease exists, parses expiry from lease value, finds message key by scanning messages CF, atomically deletes message + lease + lease_expiry via WriteBatch
• Implement Ack gRPC RPC with UUID parsing and input validation
• Add parse_expiry_from_lease_value() to keys module
• Acking unknown or already-acked message returns NOT_FOUND (idempotent)

Test plan

• Full lifecycle: enqueue -> lease -> ack -> verify message and lease deleted from storage
• Ack unknown message returns MessageNotFound
• Double ack returns MessageNotFound on second call (idempotent)
• Ack without lease returns error
• 44 tests pass, clippy clean, cargo fmt clean

---

Summary by cubic

Implements end-to-end message acknowledgment for Story 1.8. On ack, the scheduler atomically deletes the message, lease, and lease expiry; unknown, double-acked, or missing messages/queues return NOT_FOUND.

• New Features

  • Scheduler: handle_ack verifies lease, parses expiry, finds the message, and deletes message/lease/lease_expiry in one WriteBatch.
  • gRPC: Ack RPC with input validation and UUID parsing; maps AckError to NOT_FOUND or INTERNAL.
  • Keys: added parse_expiry_from_lease_value helper.

• Bug Fixes

  • Use StorageError::CorruptData for corrupt lease values.

^{Written for commit 3d21a80. Summary will update on new commits.}

[cubic-dev-ai]

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