Store delivery and archive times without the monotonic reading (closes #529)

finished_at, archived_at and the updated_at stamp a re-dispatch
writes came from a bare time.Now(). The SQLite driver stores a time
as time.Time.String(), which appends the monotonic clock reading
(" m=+…") to a time read from time.Now(). All three now come from
GORM's own clock, NowFunc(), which stamps created_at, so they are
stored in the same form. The tests for each write read the stored
text back.

Model: opus-5-5

internal/delivery/engine.go

@@ -1701,8 +1701,11 @@ func writeDeliveryStatus(
 			database.DeliveryStatusFailed,
 		}).
 		Updates(map[string]any{
-			"status":      status,
-			"finished_at": time.Now(),
+			"status": status,
+			// GORM's clock, the one that stamps created_at. A bare
+			// time.Now() carries a monotonic clock reading, which the
+			// driver would store as an " m=+…" suffix.
+			"finished_at": tx.NowFunc(),
 		})
 	if res.Error != nil || res.RowsAffected == 0 {
 		return res.Error
@@ -1906,12 +1909,13 @@ func (e *Engine) takeForRedispatch(
 		return false
 	}
 
+	// GORM's clock, as writeDeliveryStatus explains.
 	res := webhookDB.
 		Model(&database.Delivery{}).
 		Where(
 			"id = ? AND status = ?", deliveryID, eligible,
 		).
-		UpdateColumn("updated_at", time.Now())
+		UpdateColumn("updated_at", webhookDB.NowFunc())
 
 	if res.Error != nil {
 		e.log.Error(