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/webhook_db_manager.go

@@ -35,7 +35,8 @@ var errInvalidCachedDBType = errors.New(
 
 // WebhookDBManager manages per-webhook SQLite database files
 // for event storage. Each webhook gets its own dedicated
-// database containing Events, Deliveries, and DeliveryResults.
+// database containing Events, Deliveries, DeliveryResults and the
+// running Totals of them.
 // Database connections are opened lazily and cached.
 type WebhookDBManager struct {
 	dataDir string
@@ -294,7 +295,7 @@ func (m *WebhookDBManager) openDB(
 
 	// Run migrations for event-tier models only
 	err = db.AutoMigrate(
-		&Event{}, &Delivery{}, &DeliveryResult{},
+		&Event{}, &Delivery{}, &DeliveryResult{}, &Totals{},
 	)
 	if err != nil {
 		_ = sqlDB.Close()
@@ -305,6 +306,17 @@ func (m *WebhookDBManager) openDB(
 		)
 	}
 
+	// A new database gets its row of running totals, all zero.
+	err = db.FirstOrCreate(&Totals{}).Error
+	if err != nil {
+		_ = sqlDB.Close()
+
+		return nil, fmt.Errorf(
+			"creating running totals for webhook database %s: %w",
+			webhookID, err,
+		)
+	}
+
 	m.log.Info(
 		"opened per-webhook database",
 		"webhook_id", webhookID,