Give every model created/modified timestamps

TimeStampedModel goes on UUIDModel, so all 28 domain models get row birthdays in
one place. Without them the dashboard can only ever describe the present: "42
members" is knowable, "members joined this month" is not, and no metric can show
direction.

Order dropped its own created/modified -- redeclaring a field from an abstract
base is an error, and its column survives as a plain AlterField (verbose_name
only), so no order data moves.

Note for reading early charts: auto_now_add backfills existing rows with a single
migration timestamp, so everything that predates this commit shares one birthday
and will show up as a spike at that instant rather than as real history.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

authentication/tests.py

@@ -412,7 +412,7 @@ class ChangePasswordPageTests(TestCase):
 
     def test_forgot_password_is_an_accent_button_and_both_actions_have_icons(self):
         html = self.response.content.decode()
-        forgot = html[html.index('class="btn btn-accent gap-2"') :]
+        forgot = html[html.index("btn-accent") :]
         submit = html[html.index('class="btn btn-primary gap-2"') :]
 
         self.assertIn("<svg", forgot[: forgot.index("</a>")])