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>

shop/models.py

@@ -142,8 +142,8 @@ class Order(ClubScopedModel):
     status = models.CharField(_("status"), max_length=255, choices=OrderStatus.choices, default=OrderStatus.PENDING)
     total = models.DecimalField(_("total"), max_digits=10, decimal_places=2)
 
-    created = models.DateTimeField(_("created at"), auto_now_add=True)
-    modified = models.DateTimeField(_("modified"), auto_now=True)
+    # `created`/`modified` come from TimeStampedModel — declaring them here as well
+    # would clash with the abstract base.
 
     class Meta:
         verbose_name = _("order")