feat: implement persistent cart with database storage and proper exception handling

Implemented a complete shopping cart system with database persistence,
replacing the previous stub implementation. The cart now properly stores
items across sessions and follows the project's architecture patterns.

Database Changes:
- Add cart_items table with vendor_id, session_id, product_id, quantity, price_at_add
- Create unique constraint to prevent duplicate items per session
- Add indexes for session lookups and old cart cleanup
- Run migration a2064e1dfcd4 to create cart_items table

New Models & Schemas:
- models/database/cart.py: CartItem SQLAlchemy model with relationships
- models/schema/cart.py: Pydantic schemas for requests/responses
  * AddToCartRequest, UpdateCartItemRequest
  * CartResponse, CartItemResponse, CartOperationResponse, ClearCartResponse

Exception Handling:
- app/exceptions/cart.py: Cart-specific exceptions following project patterns
  * CartItemNotFoundException - item not found in cart
  * InsufficientInventoryForCartException - not enough inventory for cart operation
  * InvalidCartQuantityException - invalid quantity validation
  * CartValidationException - general cart validation
  * EmptyCartException - operations on empty cart
  * ProductNotAvailableForCartException - product unavailable
- Updated app/exceptions/__init__.py to export cart exceptions

Service Layer:
- Implement cart_service.get_cart() - fetch cart from database with totals
- Implement cart_service.add_to_cart() - create or update cart items with inventory checks
- Implement cart_service.update_cart_item() - update quantity with validation
- Implement cart_service.remove_from_cart() - delete cart item
- Implement cart_service.clear_cart() - remove all items for session
- Replace generic exceptions with cart-specific ones
- Fix InsufficientInventoryException usage (was using wrong parameters)

API Layer:
- Update app/api/v1/shop/cart.py to use Pydantic schemas
- Add response_model declarations to all endpoints
- Add return type hints for type safety
- Convert service dict responses to Pydantic models

Features:
- Cart items persist in database across server restarts
- Inventory validation before adding/updating items
- Price captured at time of adding to cart
- Duplicate items update quantity instead of creating new entries
- Full CRUD operations with proper error handling
- Type-safe API with auto-generated OpenAPI documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/api/v1/shop/cart.py

@@ -10,40 +10,31 @@ No authentication required - uses session ID for cart tracking.
 import logging
 from fastapi import APIRouter, Depends, Path, Body, Request, HTTPException
 from sqlalchemy.orm import Session
-from pydantic import BaseModel, Field
 
 from app.core.database import get_db
 from app.services.cart_service import cart_service
+from models.schema.cart import (
+    AddToCartRequest,
+    UpdateCartItemRequest,
+    CartResponse,
+    CartOperationResponse,
+    ClearCartResponse,
+)
 
 router = APIRouter()
 logger = logging.getLogger(__name__)
 
 
-# ============================================================================
-# REQUEST/RESPONSE SCHEMAS
-# ============================================================================
-
-class AddToCartRequest(BaseModel):
-    """Request model for adding to cart."""
-    product_id: int = Field(..., description="Product ID to add", gt=0)
-    quantity: int = Field(1, ge=1, description="Quantity to add")
-
-
-class UpdateCartItemRequest(BaseModel):
-    """Request model for updating cart item."""
-    quantity: int = Field(..., ge=1, description="New quantity")
-
-
 # ============================================================================
 # CART ENDPOINTS
 # ============================================================================
 
-@router.get("/cart/{session_id}")
+@router.get("/cart/{session_id}", response_model=CartResponse)
 def get_cart(
     request: Request,
     session_id: str = Path(..., description="Shopping session ID"),
     db: Session = Depends(get_db),
-):
+) -> CartResponse:
     """
     Get shopping cart contents for current vendor.
 
@@ -62,8 +53,8 @@ def get_cart(
             detail="Vendor not found. Please access via vendor domain/subdomain/path."
         )
 
-    logger.debug(
-        f"[SHOP_API] get_cart for session {session_id}",
+    logger.info(
+        f"[SHOP_API] get_cart for session {session_id}, vendor {vendor.id}",
         extra={
             "vendor_id": vendor.id,
             "vendor_code": vendor.subdomain,
@@ -77,16 +68,26 @@ def get_cart(
         session_id=session_id
     )
 
-    return cart
+    logger.info(
+        f"[SHOP_API] get_cart result: {len(cart.get('items', []))} items in cart",
+        extra={
+            "session_id": session_id,
+            "vendor_id": vendor.id,
+            "item_count": len(cart.get('items', [])),
+            "total": cart.get('total', 0),
+        }
+    )
+
+    return CartResponse.from_service_dict(cart)
 
 
-@router.post("/cart/{session_id}/items")
+@router.post("/cart/{session_id}/items", response_model=CartOperationResponse)
 def add_to_cart(
     request: Request,
     session_id: str = Path(..., description="Shopping session ID"),
     cart_data: AddToCartRequest = Body(...),
     db: Session = Depends(get_db),
-):
+) -> CartOperationResponse:
     """
     Add product to cart for current vendor.
 
@@ -109,8 +110,8 @@ def add_to_cart(
             detail="Vendor not found. Please access via vendor domain/subdomain/path."
         )
 
-    logger.debug(
-        f"[SHOP_API] add_to_cart: product {cart_data.product_id}, qty {cart_data.quantity}",
+    logger.info(
+        f"[SHOP_API] add_to_cart: product {cart_data.product_id}, qty {cart_data.quantity}, session {session_id}",
         extra={
             "vendor_id": vendor.id,
             "vendor_code": vendor.subdomain,
@@ -128,17 +129,25 @@ def add_to_cart(
         quantity=cart_data.quantity
     )
 
-    return result
+    logger.info(
+        f"[SHOP_API] add_to_cart result: {result}",
+        extra={
+            "session_id": session_id,
+            "result": result,
+        }
+    )
+
+    return CartOperationResponse(**result)
 
 
-@router.put("/cart/{session_id}/items/{product_id}")
+@router.put("/cart/{session_id}/items/{product_id}", response_model=CartOperationResponse)
 def update_cart_item(
     request: Request,
     session_id: str = Path(..., description="Shopping session ID"),
     product_id: int = Path(..., description="Product ID", gt=0),
     cart_data: UpdateCartItemRequest = Body(...),
     db: Session = Depends(get_db),
-):
+) -> CartOperationResponse:
     """
     Update cart item quantity for current vendor.
 
@@ -180,16 +189,16 @@ def update_cart_item(
         quantity=cart_data.quantity
     )
 
-    return result
+    return CartOperationResponse(**result)
 
 
-@router.delete("/cart/{session_id}/items/{product_id}")
+@router.delete("/cart/{session_id}/items/{product_id}", response_model=CartOperationResponse)
 def remove_from_cart(
     request: Request,
     session_id: str = Path(..., description="Shopping session ID"),
     product_id: int = Path(..., description="Product ID", gt=0),
     db: Session = Depends(get_db),
-):
+) -> CartOperationResponse:
     """
     Remove item from cart for current vendor.
 
@@ -226,15 +235,15 @@ def remove_from_cart(
         product_id=product_id
     )
 
-    return result
+    return CartOperationResponse(**result)
 
 
-@router.delete("/cart/{session_id}")
+@router.delete("/cart/{session_id}", response_model=ClearCartResponse)
 def clear_cart(
     request: Request,
     session_id: str = Path(..., description="Shopping session ID"),
     db: Session = Depends(get_db),
-):
+) -> ClearCartResponse:
     """
     Clear all items from cart for current vendor.
 
@@ -268,4 +277,4 @@ def clear_cart(
         session_id=session_id
     )
 
-    return result
+    return ClearCartResponse(**result)