Decouple cart/market DBs: denormalize product data, AP internal inbox, OAuth scraper auth

Remove cross-DB relationships (CartItem.product, CartItem.market_place,
OrderItem.product) that break with per-service databases. Denormalize
product and marketplace fields onto cart_items/order_items at write time.

- Add AP internal inbox infrastructure (shared/infrastructure/internal_inbox*)
  for synchronous inter-service writes via HMAC-authenticated POST
- Cart inbox blueprint handles Add/Remove/Update rose:CartItem activities
- Market app sends AP activities to cart inbox instead of writing CartItem directly
- Cart services use denormalized columns instead of cross-DB hydration/joins
- Add marketplaces-by-ids data endpoint to market service
- Alembic migration adds denormalized columns to cart_items and order_items
- Add OAuth device flow auth to market scraper persist_api (artdag client pattern)

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

cart/bp/cart/services/total.py

@@ -4,10 +4,9 @@ from decimal import Decimal
 def total(cart):
     return sum(
         (
-            Decimal(str(item.product.special_price or item.product.regular_price))
+            Decimal(str(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
+        if (item.product_special_price or item.product_regular_price) is not None
     )
-