feat: message enqueue rpc (story 1.6)

[vieiralucas]

Summary

• Implement Enqueue gRPC RPC in new HotPathService (FilaService trait)
• Add handle_enqueue in scheduler: validates queue exists, constructs composite message key, persists to RocksDB messages CF
• Stub Lease, Ack, Nack RPCs as UNIMPLEMENTED for future stories
• Register FilaServiceServer alongside FilaAdminServer in gRPC server builder

Test plan

• Enqueue to non-existent queue returns QueueNotFound error
• Enqueue persists message and verifies round-trip via get_message
• Bulk enqueue 100 messages: all unique UUIDv7 IDs, time-ordered, all persisted
• Existing scheduler/broker tests updated to create queue before enqueue (queue validation now enforced)
• 35 tests pass, clippy clean, cargo fmt clean

---

Summary by cubic

Implements Story 1.6 by adding the Enqueue gRPC RPC so producers can write messages to named queues with durable RocksDB storage. Validates queue existence, rejects empty queue names, assigns UUIDv7 IDs, persists via composite keys, and returns the ID.

• New Features

  • Added HotPathService (FilaService) with Enqueue; Lease/Ack/Nack remain UNIMPLEMENTED.
  • Scheduler handle_enqueue verifies queue, builds message_key, persists to the messages CF, and replies with the message ID.
  • Messages use UUIDv7 IDs, default fairness key "default", nanosecond timestamps; headers and payload preserved.
  • Registered FilaServiceServer alongside FilaAdminServer.
  • Error mapping: empty queue -> invalid_argument; QueueNotFound -> not_found; Storage -> internal.

• Refactors

  • Replaced #[allow(dead_code)] with #[cfg(test)] for test-only helpers and storage key prefix/lease functions.

^{Written for commit b188778. Summary will update on new commits.}

[cubic-dev-ai]

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