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/retention.go

@@ -267,55 +267,101 @@ func retentionCutoff(
 
 // reapExpired hard-deletes, in foreign-key-safe order, the delivery
 // results, deliveries, and events associated with events older than
-// cutoff. Deletes are unscoped so rows are physically removed rather
-// than soft-deleted, reclaiming disk. It returns the number of events
-// deleted.
+// cutoff, and adds what it deleted to the running totals, all in one
+// transaction. Deletes are unscoped so rows are physically removed
+// rather than soft-deleted, reclaiming disk. It returns the number of
+// events deleted.
 func reapExpired(db *gorm.DB, cutoff time.Time) (int64, error) {
+	var removed Totals
+
+	err := db.Transaction(func(tx *gorm.DB) error {
+		var err error
+
+		removed, err = deleteExpired(tx, cutoff)
+		if err != nil {
+			return err
+		}
+
+		return AddTotals(tx, removed)
+	})
+	if err != nil {
+		return 0, err
+	}
+
+	return removed.EventsRemoved, nil
+}
+
+// deleteExpired runs reapExpired's deletes and returns how many
+// events, deliveries and failed deliveries they removed.
+func deleteExpired(tx *gorm.DB, cutoff time.Time) (Totals, error) {
+	var removed Totals
+
 	// Fresh subqueries are built per statement to avoid reusing a
 	// mutated builder across executions.
 	expiredEventIDs := func() *gorm.DB {
-		return db.Model(&Event{}).
+		return tx.Model(&Event{}).
 			Select("id").
 			Where("created_at < ?", cutoff)
 	}
 	expiredDeliveryIDs := func() *gorm.DB {
-		return db.Model(&Delivery{}).
+		return tx.Model(&Delivery{}).
 			Select("id").
 			Where("event_id IN (?)", expiredEventIDs())
 	}
 
 	// 1. Delivery results whose delivery belongs to an expired event.
-	res := db.Unscoped().
+	res := tx.Unscoped().
 		Where("delivery_id IN (?)", expiredDeliveryIDs()).
 		Delete(&DeliveryResult{})
 	if res.Error != nil {
-		return 0, fmt.Errorf(
+		return removed, fmt.Errorf(
 			"deleting expired delivery results: %w",
 			res.Error,
 		)
 	}
 
-	// 2. Deliveries belonging to an expired event.
-	del := db.Unscoped().
+	// 2. Deliveries belonging to an expired event, after counting the
+	// failed ones among them. The status is tested in the select list
+	// rather than the WHERE clause: there, SQLite would read every
+	// failed delivery the webhook has through the status index,
+	// instead of only the expired ones through the event_id index.
+	var failed struct{ Count int64 }
+
+	err := tx.Unscoped().Model(&Delivery{}).
+		Select("count(CASE WHEN status = ? THEN 1 END) AS count",
+			DeliveryStatusFailed).
+		Where("event_id IN (?)", expiredEventIDs()).
+		Take(&failed).Error
+	if err != nil {
+		return removed, fmt.Errorf(
+			"counting expired failed deliveries: %w", err,
+		)
+	}
+
+	del := tx.Unscoped().
 		Where("event_id IN (?)", expiredEventIDs()).
 		Delete(&Delivery{})
 	if del.Error != nil {
-		return 0, fmt.Errorf(
+		return removed, fmt.Errorf(
 			"deleting expired deliveries: %w",
 			del.Error,
 		)
 	}
 
 	// 3. The expired events themselves.
-	ev := db.Unscoped().
+	ev := tx.Unscoped().
 		Where("created_at < ?", cutoff).
 		Delete(&Event{})
 	if ev.Error != nil {
-		return 0, fmt.Errorf(
+		return removed, fmt.Errorf(
 			"deleting expired events: %w",
 			ev.Error,
 		)
 	}
 
-	return ev.RowsAffected, nil
+	removed.EventsRemoved = ev.RowsAffected
+	removed.DeliveriesRemoved = del.RowsAffected
+	removed.FailuresRemoved = failed.Count
+
+	return removed, nil
 }