style: apply black and isort formatting across entire codebase

- Standardize quote style (single to double quotes)
- Reorder and group imports alphabetically
- Fix line breaks and indentation for consistency
- Apply PEP 8 formatting standards

Also updated Makefile to exclude both venv and .venv from code quality checks.

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

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

app/exceptions/cart.py

@@ -4,11 +4,9 @@ Shopping cart specific exceptions.
 """
 
 from typing import Optional
-from .base import (
-    ResourceNotFoundException,
-    ValidationException,
-    BusinessLogicException
-)
+
+from .base import (BusinessLogicException, ResourceNotFoundException,
+                   ValidationException)
 
 
 class CartItemNotFoundException(ResourceNotFoundException):
@@ -19,22 +17,16 @@ class CartItemNotFoundException(ResourceNotFoundException):
             resource_type="CartItem",
             identifier=str(product_id),
             message=f"Product {product_id} not found in cart",
-            error_code="CART_ITEM_NOT_FOUND"
+            error_code="CART_ITEM_NOT_FOUND",
         )
-        self.details.update({
-            "product_id": product_id,
-            "session_id": session_id
-        })
+        self.details.update({"product_id": product_id, "session_id": session_id})
 
 
 class EmptyCartException(ValidationException):
     """Raised when trying to perform operations on an empty cart."""
 
     def __init__(self, session_id: str):
-        super().__init__(
-            message="Cart is empty",
-            details={"session_id": session_id}
-        )
+        super().__init__(message="Cart is empty", details={"session_id": session_id})
         self.error_code = "CART_EMPTY"
 
 
@@ -82,7 +74,9 @@ class InsufficientInventoryForCartException(BusinessLogicException):
 class InvalidCartQuantityException(ValidationException):
     """Raised when cart quantity is invalid."""
 
-    def __init__(self, quantity: int, min_quantity: int = 1, max_quantity: Optional[int] = None):
+    def __init__(
+        self, quantity: int, min_quantity: int = 1, max_quantity: Optional[int] = None
+    ):
         if quantity < min_quantity:
             message = f"Quantity must be at least {min_quantity}"
         elif max_quantity and quantity > max_quantity: