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

@@ -199,6 +199,40 @@ func TestStatisticsQueriesUseTheirIndexes(t *testing.T) {
 			"(deleted_at=? AND created_at>?)")
 }
 
+// TestResubmitCountUsesItsIndex does the same for the event log's count
+// of the events resubmitted from each of a page's events (resubmitCounts
+// in the handlers). It passes a full page of 25 ids: with an index on
+// resubmitted_from_id alone, SQLite uses it for three ids and turns to
+// the deleted_at index from five.
+func TestResubmitCountUsesItsIndex(t *testing.T) {
+	t.Parallel()
+
+	mgr, lc := setupTestWebhookDBManager(t)
+	ctx := context.Background()
+	require.NoError(t, lc.Start(ctx))
+
+	defer func() { require.NoError(t, lc.Stop(ctx)) }()
+
+	db, err := mgr.GetDB(uuid.New().String())
+	require.NoError(t, err)
+
+	dry := db.Session(&gorm.Session{DryRun: true})
+
+	page := make([]string, 25)
+	for i := range page {
+		page[i] = uuid.New().String()
+	}
+
+	var counts []struct{ Total int }
+
+	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(&counts),
+		"idx_events_resubmitted_from_id "+
+			"(resubmitted_from_id=? AND deleted_at=?)")
+}
+
 // assertPlanUses asserts that SQLite's plan for a statement GORM built
 // in a dry run, run with the same SQL and arguments GORM would send,
 // names each of the given indexes.