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/inflight_test.go

@@ -462,7 +462,8 @@ func TestScheduledRetryIsNotSweptDuringBackoff(t *testing.T) {
 
 // TestRedispatchStampsTheRow pins the cadence control: a stranded
 // delivery that has just been handed out is not selected again by the
-// next tick a minute later.
+// next tick a minute later. The stamp is stored in the same form as
+// created_at.
 func TestRedispatchStampsTheRow(t *testing.T) {
 	t.Parallel()
 
@@ -493,4 +494,13 @@ func TestRedispatchStampsTheRow(t *testing.T) {
 		"a re-dispatched delivery must be stamped so the "+
 			"next tick does not select it again",
 	)
+
+	var createdAt, updatedAt string
+
+	require.NoError(t, s.WebhookDB.Raw(
+		`SELECT CAST(created_at AS TEXT), CAST(updated_at AS TEXT)
+		FROM deliveries WHERE id = ?`, d.ID,
+	).Row().Scan(&createdAt, &updatedAt))
+	assertStoredTime(t, createdAt)
+	assertStoredTime(t, updatedAt)
 }