fix: database target writes to dedicated archive table

The "database" target type now writes events to a separate
archived_events table instead of just marking the delivery as done.
This table persists independently of internal event retention/pruning,
allowing the data to be consumed by external systems or preserved
indefinitely.

New ArchivedEvent model copies the full event payload (method, headers,
body, content_type) along with webhook/entrypoint/event/target IDs.

internal/delivery/engine.go

@@ -244,7 +244,32 @@ func (e *Engine) deliverRetry(_ context.Context, d *database.Delivery) {
 }
 
 func (e *Engine) deliverDatabase(d *database.Delivery) {
-	// The event is already stored in the database; mark as delivered.
+	// Write the event to the dedicated archived_events table. This table
+	// persists independently of internal event retention/pruning, so the
+	// data remains available for external consumption even after the
+	// original event is cleaned up.
+	archived := &database.ArchivedEvent{
+		WebhookID:    d.Event.WebhookID,
+		EntrypointID: d.Event.EntrypointID,
+		EventID:      d.EventID,
+		TargetID:     d.TargetID,
+		Method:       d.Event.Method,
+		Headers:      d.Event.Headers,
+		Body:         d.Event.Body,
+		ContentType:  d.Event.ContentType,
+	}
+
+	if err := e.database.DB().Create(archived).Error; err != nil {
+		e.log.Error("failed to archive event",
+			"delivery_id", d.ID,
+			"event_id", d.EventID,
+			"error", err,
+		)
+		e.recordResult(d, 1, false, 0, "", err.Error(), 0)
+		e.updateDeliveryStatus(d, database.DeliveryStatusFailed)
+		return
+	}
+
 	e.recordResult(d, 1, true, 0, "", "", 0)
 	e.updateDeliveryStatus(d, database.DeliveryStatusDelivered)
 }