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/handlers/webhook.go

@@ -252,11 +252,12 @@ func requestEventSource(
 	}
 }
 
-// createAndFanOut writes the event and one pending delivery per target
-// in a single transaction, then hands the tasks to the delivery
-// engine. It is the only path by which an event and its deliveries are
-// created, so a resubmitted event is retried, SSRF-guarded and
-// circuit-broken exactly as a received one is.
+// createAndFanOut writes the event and one pending delivery per target,
+// and adds them to the webhook's running totals, in a single
+// transaction, then hands the tasks to the delivery engine. It is the
+// only path by which an event and its deliveries are created, so a
+// resubmitted event is retried, SSRF-guarded and circuit-broken
+// exactly as a received one is.
 //
 // The tasks are returned as well as queued, so a caller can report how
 // many targets the event went to.
@@ -296,6 +297,16 @@ func (h *Handlers) createAndFanOut(
 		return nil, nil, err
 	}
 
+	err = database.AddTotals(tx, database.Totals{
+		Events:     1,
+		Deliveries: int64(len(tasks)),
+	})
+	if err != nil {
+		tx.Rollback()
+
+		return nil, nil, err
+	}
+
 	err = tx.Commit().Error
 	if err != nil {
 		return nil, nil, fmt.Errorf(