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

internal/database/event_tier_indexes_test.go

@@ -93,6 +93,7 @@ func TestEventTierQueriesUseTheirIndexes(t *testing.T) {
 		deliveries []database.Delivery
 		results    []database.DeliveryResult
 		depths     []struct{ Depth int }
+		resubmits  []struct{ Total int }
 		removed    []database.TargetTotals
 	)
 
@@ -116,13 +117,29 @@ func TestEventTierQueriesUseTheirIndexes(t *testing.T) {
 		}).Group("target_id, status").Find(&depths), byStatus)
 
 	// The event log: each event's deliveries, then their attempts
-	// (loadEventsWithDeliveries, loadDeliveryResults).
+	// (loadEventsWithDeliveries, loadDeliveryResults), and how many
+	// events were resubmitted from each of the page's (resubmitCounts).
 	assertPlanUses(t, db, dry.Where("event_id = ?", ids[0]).
 		Find(&deliveries), byEvent)
 	assertPlanUses(t, db, dry.Where("delivery_id IN ?", ids).
 		Order("attempt_num ASC").Find(&results),
 		"idx_delivery_results_delivery_id (delivery_id=? AND deleted_at=?)")
 
+	// The resubmit count gets a full page of 25 ids: with an index on
+	// resubmitted_from_id alone, SQLite still uses it for three ids and
+	// turns to the deleted_at index only from five.
+	page := make([]string, 25)
+	for i := range page {
+		page[i] = uuid.New().String()
+	}
+
+	assertPlanUses(t, db, dry.Model(&database.Event{}).
+		Select("resubmitted_from_id, count(*) AS total").
+		Where("resubmitted_from_id IN ?", page).
+		Group("resubmitted_from_id").Find(&resubmits),
+		"idx_events_resubmitted_from_id "+
+			"(resubmitted_from_id=? AND deleted_at=?)")
+
 	// Retention (reapExpired, deleteEvents): one batch of expired
 	// events, then their attempts, deliveries and the events.
 	var expired []string