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_totals.go

@@ -0,0 +1,73 @@
+package database
+
+import (
+	"fmt"
+
+	"gorm.io/gorm"
+)
+
+// Totals is the single row of running totals in a webhook's event
+// database. It is what keeps the webhook page's lifetime figures right
+// after retention has removed the rows they count, and what lets the
+// page show them without counting every row.
+//
+// Storing an event, creating a delivery and failing a delivery each
+// add one, and retention adds what it deletes to the Removed columns.
+// Every addition goes through AddTotals, in the transaction that
+// writes or deletes the rows it counts.
+type Totals struct {
+	ID int64 `gorm:"primaryKey"`
+
+	Events     int64 `gorm:"not null"`
+	Deliveries int64 `gorm:"not null"`
+	Failures   int64 `gorm:"not null"`
+
+	EventsRemoved     int64 `gorm:"not null"`
+	DeliveriesRemoved int64 `gorm:"not null"`
+	FailuresRemoved   int64 `gorm:"not null"`
+}
+
+// TableName names the table AddTotals updates.
+func (Totals) TableName() string {
+	return "totals"
+}
+
+// EventsWithinRetention is how many of the webhook's events are still
+// stored.
+func (t Totals) EventsWithinRetention() int64 {
+	return t.Events - t.EventsRemoved
+}
+
+// DeliveriesWithinRetention is how many of the webhook's deliveries
+// are still stored.
+func (t Totals) DeliveriesWithinRetention() int64 {
+	return t.Deliveries - t.DeliveriesRemoved
+}
+
+// FailuresWithinRetention is how many of the webhook's failed
+// deliveries are still stored.
+func (t Totals) FailuresWithinRetention() int64 {
+	return t.Failures - t.FailuresRemoved
+}
+
+// AddTotals adds each count in add to the webhook's running totals.
+// Call it on the transaction that writes or deletes the rows it
+// counts, so the totals change exactly when those rows do.
+func AddTotals(tx *gorm.DB, add Totals) error {
+	err := tx.Exec(
+		`UPDATE totals SET
+			events = events + ?,
+			deliveries = deliveries + ?,
+			failures = failures + ?,
+			events_removed = events_removed + ?,
+			deliveries_removed = deliveries_removed + ?,
+			failures_removed = failures_removed + ?`,
+		add.Events, add.Deliveries, add.Failures,
+		add.EventsRemoved, add.DeliveriesRemoved, add.FailuresRemoved,
+	).Error
+	if err != nil {
+		return fmt.Errorf("adding to running totals: %w", err)
+	}
+
+	return nil
+}