Add a statistics pane to the webhook page (closes #368)

Each webhook's event database keeps one row of running totals: events,
deliveries and failures, and how many of each retention removed.
Storing an event, creating a delivery, a delivery becoming failed and
the retention sweep each update it in the transaction that writes or
deletes the rows it counts. Deliveries get a finished_at column, the
last column of the status index, so the last-10-minutes and
last-24-hours figures are index-range counts. The pane is its own
template, included at the top of the page.

The schema changes in place with nothing back-filled, so an existing
database must be recreated.

Model: opus-5-5

internal/database/event_tier_indexes_test.go

@@ -123,7 +123,7 @@ func TestEventTierQueriesUseTheirIndexes(t *testing.T) {
 		Order("attempt_num ASC").Find(&results),
 		"idx_delivery_results_delivery_id (delivery_id=? AND deleted_at=?)")
 
-	// Retention's three deletes (reapExpired), whose subqueries are built
+	// Retention's deletes (deleteExpired), whose subqueries are built
 	// afresh for each statement as it builds them.
 	expiredEventIDs := func() *gorm.DB {
 		return dry.Model(&database.Event{}).Select("id").
@@ -135,6 +135,14 @@ func TestEventTierQueriesUseTheirIndexes(t *testing.T) {
 			Select("id").Where("event_id IN (?)", expiredEventIDs()),
 	).Delete(&database.DeliveryResult{}),
 		"idx_delivery_results_delivery_id (delivery_id=?)", byEvent, byAge)
+	var failed struct{ Count int64 }
+
+	assertPlanUses(t, db, dry.Unscoped().Model(&database.Delivery{}).
+		Select("count(CASE WHEN status = ? THEN 1 END) AS count",
+			database.DeliveryStatusFailed).
+		Where("event_id IN (?)", expiredEventIDs()).
+		Take(&failed),
+		"idx_deliveries_event_id (event_id=?)", byAge)
 	assertPlanUses(t, db, dry.Unscoped().Where(
 		"event_id IN (?)", expiredEventIDs(),
 	).Delete(&database.Delivery{}),
@@ -142,6 +150,36 @@ func TestEventTierQueriesUseTheirIndexes(t *testing.T) {
 	assertPlanUses(t, db, dry.Unscoped().Where(
 		"created_at < ?", cutoff,
 	).Delete(&database.Event{}), "idx_events_created_at (created_at<?)")
+
+	// The webhook page's statistics (readEventStats in the handlers):
+	// deliveries in progress, deliveries finished and events received
+	// since a time, and the newest event, which must come straight
+	// off an index rather than from sorting every event.
+	var (
+		count  int64
+		newest []time.Time
+	)
+
+	assertPlanUses(t, db, dry.Model(&database.Delivery{}).
+		Where("status IN ?", []database.DeliveryStatus{
+			database.DeliveryStatusPending,
+			database.DeliveryStatusRetrying,
+		}).Count(&count), byStatus)
+	assertPlanUses(t, db, dry.Model(&database.Delivery{}).
+		Where("status = ? AND finished_at >= ?",
+			database.DeliveryStatusFailed, cutoff).Count(&count),
+		"idx_deliveries_status "+
+			"(status=? AND deleted_at=? AND finished_at>?)")
+	assertPlanUses(t, db, dry.Model(&database.Event{}).
+		Where("created_at >= ?", cutoff).Count(&count),
+		"idx_events_deleted_at_created_at "+
+			"(deleted_at=? AND created_at>?)")
+
+	newestEvent := dry.Model(&database.Event{}).
+		Order("created_at DESC").Limit(1).Pluck("created_at", &newest)
+	assertPlanUses(t, db, newestEvent,
+		"idx_events_deleted_at_created_at (deleted_at=?)")
+	assert.NotContains(t, queryPlan(t, db, newestEvent), "TEMP B-TREE")
 }
 
 // assertPlanUses asserts that SQLite's plan for a statement GORM built
@@ -152,6 +190,18 @@ func assertPlanUses(
 ) {
 	t.Helper()
 
+	plan := queryPlan(t, db, built)
+
+	for _, index := range indexes {
+		assert.Contains(t, plan, index, built.Statement.SQL.String())
+	}
+}
+
+// queryPlan returns SQLite's plan for a statement GORM built in a dry
+// run, run with the same SQL and arguments GORM would send.
+func queryPlan(t *testing.T, db, built *gorm.DB) string {
+	t.Helper()
+
 	var plan []struct{ Detail string }
 
 	require.NoError(t, db.Raw(
@@ -159,8 +209,5 @@ func assertPlanUses(
 		built.Statement.Vars...,
 	).Scan(&plan).Error)
 
-	for _, index := range indexes {
-		assert.Contains(t, fmt.Sprint(plan), index,
-			built.Statement.SQL.String())
-	}
+	return fmt.Sprint(plan)
 }