Fix: Auto-calculate SlotEntry.order when not provided via API

The SlotEntry model's `order` field was defined with `blank=True` but no
default value, causing 500 Internal Server Error when creating entries
via /api/v2/slot-entry/ without specifying an order.

Changes:
- Add `null=True` to allow Python None value before save
- Auto-calculate order in save() method as max(existing_orders) + 1

This matches the behavior of similar models (Day, Slot) which have
default=1 for their order fields.

Fixes: IntegrityError "null value in column 'order' violates not-null constraint"

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

wger/manager/models/slot_entry.py

@@ -140,6 +140,7 @@ class SlotEntry(models.Model):
 
     order = models.PositiveIntegerField(
         blank=True,
+        null=True,
         db_index=True,
     )
 
@@ -213,6 +214,12 @@ class SlotEntry(models.Model):
                 )
             if not self.weight_rounding:
                 self.weight_rounding = self.slot.day.routine.user.userprofile.weight_rounding
+
+            # Auto-calculate order if not provided
+            if self.order is None:
+                max_order = self.slot.entries.aggregate(models.Max('order'))['order__max']
+                self.order = (max_order or 0) + 1
+
         return super().save(*args, **kwargs)
 
     def get_owner_object(self):