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/service/fetch.rs

@@ -1,5 +1,9 @@
+use activitypub_federation::fetch::object_id::ObjectId;
 use url::Url;
 
+use crate::actors::DbActor;
+use crate::repository::RemoteActor;
+
 use super::ActivityPubService;
 
 impl ActivityPubService {
@@ -17,4 +21,42 @@ impl ActivityPubService {
         .map_err(|e| anyhow::anyhow!("{e}"))?;
         Ok(res.object)
     }
+
+    /// Get a cached remote actor, re-fetching from origin if stale.
+    ///
+    /// Returns `None` if the actor has never been seen. Staleness is
+    /// determined by `actor_cache_ttl_secs` (builder config).
+    pub async fn get_or_refresh_remote_actor(
+        &self,
+        actor_url: &str,
+    ) -> anyhow::Result<Option<RemoteActor>> {
+        let data = self.federation_config.to_request_data();
+        let cached = data.actor_repo.get_remote_actor(actor_url).await?;
+        if let Some(ref actor) = cached {
+            let is_fresh = actor
+                .fetched_at
+                .map(|t| {
+                    let age = chrono::Utc::now().signed_duration_since(t);
+                    age < chrono::Duration::from_std(data.actor_cache_ttl).unwrap_or_default()
+                })
+                .unwrap_or(true);
+            if is_fresh {
+                return Ok(cached);
+            }
+        }
+        let url = match Url::parse(actor_url) {
+            Ok(u) => u,
+            Err(_) => return Ok(cached),
+        };
+        match ObjectId::<DbActor>::from(url)
+            .dereference_forced(&data)
+            .await
+        {
+            Ok(_) => Ok(data.actor_repo.get_remote_actor(actor_url).await?),
+            Err(e) => {
+                tracing::warn!(actor_url, error = %e, "re-fetch failed, using stale cache");
+                Ok(cached)
+            }
+        }
+    }
 }