feat: implement federation post/connections backfill schedulers

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.

crates/bootstrap/src/factory.rs

@@ -86,6 +86,7 @@ pub async fn build(cfg: &Config) -> Infrastructure {
             "thoughts".to_string(),
             cfg.debug,
             None,
+            Arc::new(PgRemoteActorConnectionRepository::new(pool.clone())),
         )
         .await
         .expect("Failed to build ActivityPubService"),