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

Each webhook's event database keeps running totals: one row for its
events, and one row per target for that target's deliveries, delivered
and failed, each with what retention removed. Every write to them
shares the transaction of the rows it counts. 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. 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/delivery/engine.go

@@ -531,6 +531,11 @@ func (e *Engine) processRetryTask(
 		return
 	}
 
+	// Set before anything below can fail the delivery: the failure is
+	// added to this target's totals.
+	d.EventID = task.EventID
+	d.TargetID = task.TargetID
+
 	if d.Status != database.DeliveryStatusRetrying {
 		e.log.Debug(
 			"skipping retry for delivery "+
@@ -562,8 +567,6 @@ func (e *Engine) processRetryTask(
 	}
 
 	target := buildTargetFromTask(task)
-	d.EventID = task.EventID
-	d.TargetID = task.TargetID
 	d.Event = event
 	d.Target = target
 
@@ -1554,8 +1557,9 @@ func (e *Engine) updateDeliveryStatus(
 	targetType database.TargetType,
 	status database.DeliveryStatus,
 ) error {
-	err := webhookDB.Model(d).
-		Update("status", status).Error
+	err := webhookDB.Transaction(func(tx *gorm.DB) error {
+		return writeDeliveryStatus(tx, d, status)
+	})
 	if err != nil {
 		return fmt.Errorf(
 			"updating delivery %s to status %s: %w",
@@ -1574,6 +1578,36 @@ func (e *Engine) updateDeliveryStatus(
 	return nil
 }
 
+// writeDeliveryStatus writes a delivery's new status. A delivery that
+// becomes delivered or failed also gets the time it finished, and is
+// added to its target's delivered or failed total. It is counted only
+// if the row was still there to update: retention may have deleted it
+// while the engine was working on it.
+func writeDeliveryStatus(
+	tx *gorm.DB,
+	d *database.Delivery,
+	status database.DeliveryStatus,
+) error {
+	if !status.Terminal() {
+		return tx.Model(d).Update("status", status).Error
+	}
+
+	res := tx.Model(d).Updates(map[string]any{
+		"status":      status,
+		"finished_at": time.Now(),
+	})
+	if res.Error != nil || res.RowsAffected == 0 {
+		return res.Error
+	}
+
+	add := database.TargetTotals{TargetID: d.TargetID, Delivered: 1}
+	if status == database.DeliveryStatusFailed {
+		add = database.TargetTotals{TargetID: d.TargetID, Failed: 1}
+	}
+
+	return database.AddTargetTotals(tx, add)
+}
+
 // settleStatus moves a delivery to its outcome status and reports a
 // failed write through bookkeepingFailed, which leaves the row
 // recoverable. It exists so the target call sites read as one