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/database/models.go

@@ -11,5 +11,6 @@ func (d *Database) Migrate() error {
 		&Event{},
 		&Delivery{},
 		&DeliveryResult{},
+		&ArchivedEvent{},
 	)
 }