refactor(fediverse): thread crawl one level per pass, like Mastodon
Match Mastodon's FetchReplies behaviour: crawl ONE hop of replies per pass instead
of a deep BFS. Deeper levels still fill in — a cached reply becomes a seed on the
next crawl, so its replies are pulled on a later view (Mastodon's per-status cascade).
More conservative + polite; same push-first + cache + bounded design.
- src/services/ActivityPubService.js — THREAD_MAX_DEPTH 3 → 1; comment updated.
Co-Authored-By: Claude <noreply@…>