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/schemas/__init__.py

@@ -0,0 +1,14 @@
+# app/modules/checkout/schemas/__init__.py
+"""Checkout module schemas."""
+
+from app.modules.checkout.schemas.checkout import (
+    CheckoutRequest,
+    CheckoutResponse,
+    CheckoutSessionResponse,
+)
+
+__all__ = [
+    "CheckoutRequest",
+    "CheckoutResponse",
+    "CheckoutSessionResponse",
+]

app/modules/checkout/schemas/checkout.py

@@ -0,0 +1,54 @@
+# app/modules/checkout/schemas/checkout.py
+"""
+Pydantic schemas for checkout operations.
+
+These schemas handle the conversion of a shopping cart into an order.
+"""
+
+from pydantic import BaseModel, Field
+
+
+class ShippingAddress(BaseModel):
+    """Shipping address for checkout."""
+
+    first_name: str = Field(..., min_length=1, max_length=100)
+    last_name: str = Field(..., min_length=1, max_length=100)
+    address_line_1: str = Field(..., min_length=1, max_length=255)
+    address_line_2: str | None = Field(None, max_length=255)
+    city: str = Field(..., min_length=1, max_length=100)
+    postal_code: str = Field(..., min_length=1, max_length=20)
+    country: str = Field(..., min_length=2, max_length=2)  # ISO 3166-1 alpha-2
+    phone: str | None = Field(None, max_length=20)
+
+
+class CheckoutRequest(BaseModel):
+    """Request to initiate checkout."""
+
+    session_id: str = Field(..., description="Cart session ID")
+    shipping_address: ShippingAddress = Field(..., description="Shipping address")
+    billing_same_as_shipping: bool = Field(
+        True, description="Use shipping address for billing"
+    )
+    billing_address: ShippingAddress | None = Field(
+        None, description="Billing address (if different from shipping)"
+    )
+    customer_email: str = Field(..., description="Customer email for order confirmation")
+    customer_note: str | None = Field(None, description="Optional customer note")
+
+
+class CheckoutSessionResponse(BaseModel):
+    """Response for checkout session creation."""
+
+    checkout_session_id: str = Field(..., description="Checkout session ID")
+    cart_total: float = Field(..., description="Cart total in euros")
+    item_count: int = Field(..., description="Number of items in cart")
+    requires_payment: bool = Field(..., description="Whether payment is required")
+
+
+class CheckoutResponse(BaseModel):
+    """Response for completed checkout."""
+
+    order_id: int = Field(..., description="Created order ID")
+    order_number: str = Field(..., description="Human-readable order number")
+    total: float = Field(..., description="Order total in euros")
+    message: str = Field(..., description="Confirmation message")