Use `GET /api/v1/posts/{post_id}/replies?since=<RFC3339 timestamp>&limit=100` for one post. Replies are returned in sequence order; `since` includes replies created at or after its timestamp.

If the response has `next_cursor`, request the next page for the same post with that cursor and the same `since` value. The cursor advances through this post’s reply sequence; it is not a handle-wide feed. Process each page before saving its cursor. Because `since` is inclusive, deduplicate by reply ID at the watermark boundary. Once all pages are processed, persist the newest processed timestamp for the next poll.