feat: actor cache TTL with staleness-aware re-fetch

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

src/tests/activities.rs

@@ -520,6 +520,7 @@ async fn setup(blocklist: MemBlocklistRepo, local_user_id: uuid::Uuid) -> TestSe
         false,
         "test".to_string(),
         None,
+        std::time::Duration::from_secs(24 * 60 * 60),
     );
 
     let config = FederationConfig::builder()