feat: add FollowBackfillHandler to worker

crates/worker/src/follow_backfill_handler.rs

@@ -0,0 +1,22 @@
+use std::sync::Arc;
+
+use async_trait::async_trait;
+use domain::{errors::DomainError, events::DomainEvent, ports::EventHandler};
+
+pub struct FollowBackfillHandler {
+    pub ap_service: Arc<dyn activitypub::ActivityPubPort>,
+}
+
+#[async_trait]
+impl EventHandler for FollowBackfillHandler {
+    async fn handle(&self, event: &DomainEvent) -> Result<(), DomainError> {
+        let DomainEvent::FollowAccepted { remote_actor_url, outbox_url, .. } = event else {
+            return Ok(());
+        };
+        tracing::info!(actor = %remote_actor_url, outbox = %outbox_url, "starting outbox backfill");
+        self.ap_service
+            .backfill_outbox(outbox_url, remote_actor_url)
+            .await
+            .map_err(|e| DomainError::InfrastructureError(e.to_string()))
+    }
+}