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>

models/database/cart.py

@@ -0,0 +1,46 @@
+# models/database/cart.py
+"""Cart item database model."""
+from datetime import datetime
+from sqlalchemy import Column, Float, ForeignKey, Index, Integer, String, UniqueConstraint
+from sqlalchemy.orm import relationship
+
+from app.core.database import Base
+from models.database.base import TimestampMixin
+
+
+class CartItem(Base, TimestampMixin):
+    """
+    Shopping cart items.
+
+    Stores cart items per session, vendor, and product.
+    Sessions are identified by a session_id string (from browser cookies).
+    """
+    __tablename__ = "cart_items"
+
+    id = Column(Integer, primary_key=True, index=True)
+    vendor_id = Column(Integer, ForeignKey("vendors.id"), nullable=False)
+    product_id = Column(Integer, ForeignKey("products.id"), nullable=False)
+    session_id = Column(String(255), nullable=False, index=True)
+
+    # Cart details
+    quantity = Column(Integer, nullable=False, default=1)
+    price_at_add = Column(Float, nullable=False)  # Store price when added to cart
+
+    # Relationships
+    vendor = relationship("Vendor")
+    product = relationship("Product")
+
+    # Constraints
+    __table_args__ = (
+        UniqueConstraint("vendor_id", "session_id", "product_id", name="uq_cart_item"),
+        Index("idx_cart_session", "vendor_id", "session_id"),
+        Index("idx_cart_created", "created_at"),  # For cleanup of old carts
+    )
+
+    def __repr__(self):
+        return f"<CartItem(id={self.id}, session='{self.session_id}', product_id={self.product_id}, qty={self.quantity})>"
+
+    @property
+    def line_total(self) -> float:
+        """Calculate line total."""
+        return self.price_at_add * self.quantity