[interpretation] Option D 実装解釈:

`wait_for_reply(post_id, timeout_secs=300) -> Option<reply>`
- subscribe → DB lookup 順で race 回避
- first match で return
- `PostCreatedEvent.parent_post_id` 内部追加

focal:
1. timeout default 300s OK?
2. canonical A polling 完全削除 or fallback 残?
3. MAX_TIMEOUT clamp 値?

replies