Index the event log's resubmit count with deleted_at (closes #325)

The resubmit count matches resubmitted_from_id against a page of event
ids, and GORM adds deleted_at IS NULL. With no statistics, SQLite
preferred the deleted_at index once the page held five or more ids, so
every event log page read every live event. deleted_at now joins
resubmitted_from_id in idx_events_resubmitted_from_id, as for the other
event-tier indexes, and the plan test checks the statement with a full
page of ids.

Model: opus-5-5

README.md

@@ -1876,6 +1876,7 @@ tags, so `AutoMigrate` creates them on a fresh database:
 | `deliveries`       | `event_id`, `deleted_at`    | The event log, which loads each event's deliveries, and retention, which counts and deletes the deliveries of expired events |
 | `delivery_results` | `delivery_id`, `deleted_at` | The event log, which loads the attempts of a page's deliveries, and retention, which deletes the attempts of expired events |
 | `events`           | `deleted_at`, `created_at`  | The webhook page's statistics, which count recent events |
+| `events`           | `resubmitted_from_id`, `deleted_at` | The event log, which counts the events resubmitted from each event on a page |
 | `events`           | `created_at`                | Retention, which selects expired events by age |
 
 GORM's soft delete adds `deleted_at IS NULL` to these queries; retention
@@ -1883,9 +1884,9 @@ leaves it out. SQLite keeps no statistics on these tables, and without them it
 rates the `deleted_at` index, which every live row matches, above an index on
 a column matched against several values or compared with a range. So every
 index but the last also covers `deleted_at`. It comes second, so that
-retention can use the index without it, except in `events`, where the
-statistics compare `created_at` with a range (`>=`) and SQLite narrows by a
-range only on the last column it uses.
+retention can use the index without it, except in the statistics' `events`
+index, where they compare `created_at` with a range (`>=`) and SQLite narrows
+by a range only on the last column it uses.
 
 #### Common Fields