fix: add cart services stub for product blueprint dependency

Product routes import total() from cart.services — provide a minimal
copy so the market image can start independently.

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

bp/cart/services/__init__.py

@@ -0,0 +1 @@
+from .total import total

bp/cart/services/total.py

@@ -0,0 +1,6 @@
+def total(cart):
+    return sum(
+        (item.product.special_price or item.product.regular_price) * item.quantity
+        for item in cart
+        if (item.product.special_price or item.product.regular_price) is not None
+    )