Index the event-tier columns the sweeps, event log and retention scan (closes #314)

The per-webhook tables declared no secondary indexes, so the recovery
and sweep queries (by delivery status, every minute), the event log
(deliveries by event, results by delivery) and retention (events by
age) each scanned a whole table. Add indexes through GORM model tags so
AutoMigrate creates them on a fresh and on an existing per-webhook
database. events.created_at is indexed by overriding the embedded
BaseModel field on Event alone, leaving the other tables' created_at
unindexed. A test drops the indexes from an opened database, reopens it,
and asserts the open recreated them. The README Data Model section lists
the indexes.

Model: opus-4-8

README.md

@@ -1700,6 +1700,22 @@ retries) is individually logged for full observability.
 
 **Relations:** Belongs to Delivery.
 
+#### Event-tier indexes
+
+Beyond the primary keys, the per-webhook event databases carry secondary
+indexes on the columns the background work reads by, each created by
+`AutoMigrate` on a fresh and on an existing database:
+
+| Column                         | Serves |
+| ------------------------------ | ------ |
+| `deliveries.status`            | The recovery and sweep queries that select deliveries by status once a minute |
+| `deliveries.event_id`          | Loading a page of the event log, which reads deliveries by event |
+| `delivery_results.delivery_id` | Loading a page of the event log, which reads results by delivery |
+| `events.created_at`            | Retention, which deletes events by age |
+
+The `events.resubmitted_from_id` column is also indexed, to resolve the
+resubmit relationship both ways in the event log.
+
 #### Common Fields
 
 Every entity except `Setting` includes these fields from `BaseModel`.