Undo: inner activity actor must match Undo actor
Announce/Like/Block: verify_domains_match(activity_id, actor_url)
Add: attributedTo must match actor (same as Create/Update)
schedule_actor_posts_fetch now spawns backfill_outbox in background,
fetching all pages of a remote outbox and persisting via accept_note.
schedule_connections_fetch follows AP collection next-links, resolves
profiles, and caches them in the DB. Both were no-ops ("deferred").
Add connections_repo field to ActivityPubService; wire both factories.
