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/target_database_test.go

@@ -150,6 +150,13 @@ func TestArchiveWriter_WritesRow(t *testing.T) {
 	assert.Equal(t, row.ContentType, got[0].ContentType)
 	assert.JSONEq(t, `{"hello":"world"}`, got[0].Body)
 	assert.False(t, got[0].ArchivedAt.IsZero())
+
+	var archivedAt string
+
+	require.NoError(t, w.DB().Raw(
+		"SELECT CAST(archived_at AS TEXT) FROM archived_events",
+	).Row().Scan(&archivedAt))
+	assertStoredTime(t, archivedAt)
 }
 
 func TestArchiveWriter_RecreatesAfterRemoval(