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/model_delivery.go

@@ -1,6 +1,10 @@
 package database
 
-import "gorm.io/gorm"
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
 
 // DeliveryStatus represents the status of a delivery
 type DeliveryStatus string
@@ -45,6 +49,12 @@ type Delivery struct {
 	// gives.
 	DeletedAt gorm.DeletedAt `gorm:"index:idx_deliveries_event_id,priority:2;index:idx_deliveries_status,priority:2" json:"deletedAt,omitzero"`
 
+	// FinishedAt is when the delivery became delivered or failed, and
+	// nil while it is pending or retrying. It ends the status index,
+	// so the webhook page counts the deliveries that finished in a
+	// recent window by reading that window from the index.
+	FinishedAt *time.Time `gorm:"index:idx_deliveries_status,priority:3" json:"finishedAt,omitempty"`
+
 	// Relations
 	Event           Event            `json:"event,omitzero"`
 	Target          Target           `json:"target,omitzero"`