Fix receipt IntegrityError: set zero defaults before first save

subtotal and total_amount have no default in the model, so the
first receipt.save() sent NULL to MariaDB which rejects it. Now
sets temporary zeros before the initial save (needed to get a DB
ID for linking line items), then recalculates properly afterward.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/views.py

@@ -1254,6 +1254,13 @@ def create_receipt(request):
             # Save the receipt header (but don't commit yet — need to set user)
             receipt = form.save(commit=False)
             receipt.user = request.user
+            # Set temporary zero values so the first save doesn't fail.
+            # (subtotal and total_amount have no default in the model,
+            # so they'd be NULL — which MariaDB rejects.)
+            # We'll recalculate these properly after saving line items.
+            receipt.subtotal = Decimal('0.00')
+            receipt.vat_amount = Decimal('0.00')
+            receipt.total_amount = Decimal('0.00')
             receipt.save()
 
             # Save line items — link them to this receipt