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_test.go

@@ -117,6 +117,18 @@ func seedDelivery(
 	return d
 }
 
+// assertStoredTime asserts that text, a timestamp as SQLite holds it,
+// is in the form GORM stores created_at in: the layout of
+// time.Time.String, with no " m=+…" monotonic clock reading after it.
+func assertStoredTime(t *testing.T, text string) {
+	t.Helper()
+
+	_, err := time.Parse(
+		"2006-01-02 15:04:05.999999999 -0700 MST", text,
+	)
+	assert.NoError(t, err, "stored as %q", text)
+}
+
 // httpDeliveryFixture holds the task and delivery needed
 // for HTTP delivery tests.
 type httpDeliveryFixture struct {