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

Each webhook's event database keeps running totals: one row for its
events, with when the newest arrived, and one row per target for its
deliveries, delivered and failed, each with what retention removed.
Every write to them shares the transaction of the rows it counts, and
a delivery already delivered or failed is not settled again.
Deliveries get a finished_at column; it and target_id end the status
index, so each target's deliveries finished in a window come from one
index-range query grouped by target. Retention deletes 1000 expired
events per transaction, pausing 200 ms between them so other writers
get in, and stops between them on shutdown. The pane is its own
template, its figures in tables.

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

Model: opus-5-5

internal/handlers/webhook.go

@@ -253,11 +253,12 @@ 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.
@@ -297,6 +298,15 @@ func (h *Handlers) createAndFanOut(
 		return nil, nil, err
 	}
 
+	err = database.AddEventTotals(tx, database.EventTotals{
+		Events: 1, LastEventAt: &event.CreatedAt,
+	})
+	if err != nil {
+		tx.Rollback()
+
+		return nil, nil, err
+	}
+
 	err = tx.Commit().Error
 	if err != nil {
 		return nil, nil, fmt.Errorf(
@@ -355,8 +365,9 @@ func (h *Handlers) finishWebhookResponse(
 }
 
 // buildDeliveryTasks creates one pending delivery per target in the
-// transaction and returns the tasks for the delivery engine. The
-// caller owns the transaction and rolls it back on error.
+// transaction, adds each to its target's totals, and returns the tasks
+// for the delivery engine. The caller owns the transaction and rolls
+// it back on error.
 func buildDeliveryTasks(
 	tx *gorm.DB,
 	event *database.Event,
@@ -380,6 +391,13 @@ func buildDeliveryTasks(
 			)
 		}
 
+		err = database.AddTargetTotals(tx, database.TargetTotals{
+			TargetID: targets[i].ID, Deliveries: 1,
+		})
+		if err != nil {
+			return nil, err
+		}
+
 		tasks = append(tasks, delivery.Task{
 			DeliveryID:   dlv.ID,
 			EventID:      event.ID,