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/catalog/schemas/catalog.py

@@ -0,0 +1,56 @@
+# app/modules/catalog/schemas/catalog.py
+"""
+Pydantic schemas for catalog browsing operations.
+
+These schemas are for the public storefront catalog API.
+For vendor product management, see the products module.
+"""
+
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict, Field
+
+from models.schema.inventory import InventoryLocationResponse
+from models.schema.marketplace_product import MarketplaceProductResponse
+
+
+class ProductResponse(BaseModel):
+    """Response model for a product in the catalog."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    vendor_id: int
+    marketplace_product: MarketplaceProductResponse
+    vendor_sku: str | None
+    price: float | None
+    sale_price: float | None
+    currency: str | None
+    availability: str | None
+    condition: str | None
+    is_featured: bool
+    is_active: bool
+    display_order: int
+    min_quantity: int
+    max_quantity: int | None
+    created_at: datetime
+    updated_at: datetime
+
+    # Include inventory summary
+    total_inventory: int | None = None
+    available_inventory: int | None = None
+
+
+class ProductDetailResponse(ProductResponse):
+    """Product with full inventory details."""
+
+    inventory_locations: list[InventoryLocationResponse] = []
+
+
+class ProductListResponse(BaseModel):
+    """Paginated list of products."""
+
+    products: list[ProductResponse]
+    total: int
+    skip: int
+    limit: int