Show each entrypoint's last event and event count on the webhook page (closes #393)

Each entrypoint in the webhook page's entrypoint list shows when the last event arrived on its URL (relative, with the full UTC time on hover), or "never" if none ever has, and how many events arrived on it within the webhook's retention period. The last event is kept per entrypoint in a new EntrypointTotals row in the event database, written in the transaction that stores the event, so retention leaves it in place. The count is one query per page, grouped by entrypoint, over a new events index on entrypoint_id, deleted_at, resubmitted_from_id and created_at. Resubmitted copies did not arrive on the URL and count in neither. Pre-1.0: the table and index go into the schema in place.

Model: opus-5-5

internal/database/models.go

@@ -3,7 +3,7 @@ package database
 // Migrate runs database migrations for the main application database.
 // Only configuration-tier models are stored in the main database.
 // Event-tier models (Event, Delivery, DeliveryResult, EventTotals,
-// TargetTotals) live in
+// TargetTotals, EntrypointTotals) live in
 // per-webhook dedicated databases managed by WebhookDBManager.
 func (d *Database) Migrate() error {
 	return d.db.AutoMigrate(