Adds fetched_at to RemoteActor, configurable TTL via builder
(.actor_cache_ttl_secs, default 24h), and get_or_refresh_remote_actor
helper that re-fetches stale actors from origin.
Closes#3
Bump to 0.3.1. These fields are available on DbActor at follow/ingest
time but were discarded when constructing RemoteActor. Now populated
in from_json and follow(), so consuming repos can store and return
rich actor profiles without extra queries.