feat: initialize cart app with blueprints, templates, and CI

Extract cart, order, and orders blueprints with their service layer,
templates, Dockerfile (APP_MODULE=app:app, IMAGE=cart), entrypoint,
and Gitea CI workflow from the coop monolith.

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

bp/cart/services/get_cart.py

@@ -0,0 +1,24 @@
+from sqlalchemy import select
+from sqlalchemy.orm import selectinload
+
+from models.market import CartItem
+from .identity import current_cart_identity
+
+async def get_cart(session):
+  ident = current_cart_identity()
+  
+  filters = [CartItem.deleted_at.is_(None)]
+  if ident["user_id"] is not None:
+      filters.append(CartItem.user_id == ident["user_id"])
+  else:
+      filters.append(CartItem.session_id == ident["session_id"])
+
+  result = await session.execute(
+      select(CartItem)
+      .where(*filters)
+      .order_by(CartItem.created_at.desc())
+      .options(
+          selectinload(CartItem.product),  # <-- important bit
+      )
+  )
+  return result.scalars().all()