wiki: add Async Delivery and EventPublisher page

Async-Delivery-and-EventPublisher.md

@@ -0,0 +1,74 @@
+# Async Delivery and EventPublisher
+
+By default, k-ap delivers activities and runs backfill in-process via `tokio::spawn`. For production scale, implement `EventPublisher` to route these operations through your job queue.
+
+---
+
+## Default mode
+
+No configuration needed. k-ap spawns delivery and backfill tasks with `tokio::spawn`. Delivery is retried up to `delivery_max_attempts` times with exponential backoff starting at `delivery_initial_delay_secs`.
+
+---
+
+## EventPublisher
+
+```rust
+use k_ap::{EventPublisher, FederationEvent};
+
+#[async_trait]
+impl EventPublisher for MyQueue {
+    async fn publish(&self, event: FederationEvent) -> anyhow::Result<()> {
+        match event {
+            FederationEvent::DeliveryRequested { inbox, activity, signing_actor_id } => {
+                // Persist to your job queue.
+                // Your worker calls service.deliver_to_inbox(inbox, activity, signing_actor_id).
+                self.enqueue_delivery(inbox, activity, signing_actor_id).await?;
+            }
+            FederationEvent::DeliveryFailed { inbox, error, .. } => {
+                tracing::error!(%inbox, %error, "permanent delivery failure");
+            }
+            FederationEvent::BackfillRequested { owner_user_id, follower_inbox_url } => {
+                // Persist to your job queue.
+                // Your worker calls service.run_backfill_for_follower(owner_user_id, follower_inbox_url).
+                self.enqueue_backfill(owner_user_id, follower_inbox_url).await?;
+            }
+        }
+        Ok(())
+    }
+}
+```
+
+Register in the builder:
+
+```rust
+let service = ActivityPubService::builder("https://example.com")
+    // ... repos ...
+    .event_publisher(Arc::new(my_queue))
+    .build()
+    .await?;
+```
+
+---
+
+## FederationEvent variants
+
+| Variant | Fields | When published |
+|---------|--------|----------------|
+| `DeliveryRequested` | `inbox: Url`, `activity: Value`, `signing_actor_id: Uuid` | Every outbound activity delivery |
+| `DeliveryFailed` | `inbox: Url`, `activity: Value`, `signing_actor_id: Uuid`, `error: String` | After all in-process retries exhausted |
+| `BackfillRequested` | `owner_user_id: Uuid`, `follower_inbox_url: String` | When a new follower is accepted |
+
+---
+
+## Worker methods
+
+```rust
+// Execute a single delivery — called from a DeliveryRequested worker.
+service.deliver_to_inbox(inbox, activity, signing_actor_id).await?;
+
+// Send local content to a new follower's inbox — called from a BackfillRequested worker.
+// Distinct from import_remote_outbox, which imports FROM a remote server.
+service.run_backfill_for_follower(owner_user_id, follower_inbox_url).await?;
+```
+
+`run_backfill_for_follower` paginates through the local user's post history and delivers oldest-to-newest with a small sleep between batches to avoid overwhelming the remote server.