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>

templates/_base.html

@@ -81,7 +81,7 @@
             </div>
         {% endif %}
 
-        <main class="mx-auto w-full p-4">
+        <main class="mx-auto w-full py-4 px-8">
             {% block main %}{% endblock main %}
         </main>