feat(modules): create cart, catalog, and checkout e-commerce modules

Phase 3 of storefront restructure plan - create dedicated modules for
e-commerce functionality:

- cart: Shopping cart management with storefront API routes
  - CartItem model with cents-based pricing
  - CartService for cart operations
  - Storefront routes for cart CRUD operations

- catalog: Product catalog browsing for customers
  - CatalogService for public product queries
  - Storefront routes for product listing/search/details

- checkout: Order creation from cart (placeholder)
  - CheckoutService stub for future cart-to-order conversion
  - Schemas for checkout flow

These modules separate e-commerce concerns from core platform
concerns (customer auth), enabling non-commerce platforms.

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

app/modules/checkout/services/__init__.py

@@ -0,0 +1,6 @@
+# app/modules/checkout/services/__init__.py
+"""Checkout module services."""
+
+from app.modules.checkout.services.checkout_service import checkout_service
+
+__all__ = ["checkout_service"]

app/modules/checkout/services/checkout_service.py

@@ -0,0 +1,108 @@
+# app/modules/checkout/services/checkout_service.py
+"""
+Checkout service for order creation from cart.
+
+This module provides:
+- Cart validation for checkout
+- Order creation from cart
+- Checkout session management
+
+Note: This is a placeholder service. Full implementation will
+integrate with cart, orders, and payments modules.
+"""
+
+import logging
+
+from sqlalchemy.orm import Session
+
+logger = logging.getLogger(__name__)
+
+
+class CheckoutService:
+    """Service for checkout operations."""
+
+    def validate_cart_for_checkout(
+        self, db: Session, vendor_id: int, session_id: str
+    ) -> dict:
+        """
+        Validate cart is ready for checkout.
+
+        Args:
+            db: Database session
+            vendor_id: Vendor ID
+            session_id: Cart session ID
+
+        Returns:
+            Validation result with cart summary
+
+        Raises:
+            ValidationException: If cart is not valid for checkout
+        """
+        # TODO: Implement cart validation
+        # - Check cart is not empty
+        # - Check all products are still active
+        # - Check inventory is available
+        # - Calculate totals
+        logger.info(
+            "[CHECKOUT_SERVICE] validate_cart_for_checkout",
+            extra={"vendor_id": vendor_id, "session_id": session_id},
+        )
+        raise NotImplementedError("Checkout service not yet implemented")
+
+    def create_checkout_session(
+        self, db: Session, vendor_id: int, session_id: str
+    ) -> dict:
+        """
+        Create a checkout session from cart.
+
+        Args:
+            db: Database session
+            vendor_id: Vendor ID
+            session_id: Cart session ID
+
+        Returns:
+            Checkout session data
+        """
+        # TODO: Implement checkout session creation
+        logger.info(
+            "[CHECKOUT_SERVICE] create_checkout_session",
+            extra={"vendor_id": vendor_id, "session_id": session_id},
+        )
+        raise NotImplementedError("Checkout service not yet implemented")
+
+    def complete_checkout(
+        self,
+        db: Session,
+        vendor_id: int,
+        checkout_session_id: str,
+        customer_id: int | None = None,
+    ) -> dict:
+        """
+        Complete checkout and create order.
+
+        Args:
+            db: Database session
+            vendor_id: Vendor ID
+            checkout_session_id: Checkout session ID
+            customer_id: Optional customer ID (for registered users)
+
+        Returns:
+            Created order data
+        """
+        # TODO: Implement checkout completion
+        # - Convert cart to order
+        # - Clear cart
+        # - Send confirmation email
+        logger.info(
+            "[CHECKOUT_SERVICE] complete_checkout",
+            extra={
+                "vendor_id": vendor_id,
+                "checkout_session_id": checkout_session_id,
+                "customer_id": customer_id,
+            },
+        )
+        raise NotImplementedError("Checkout service not yet implemented")
+
+
+# Create service instance
+checkout_service = CheckoutService()