refactor: modernize code quality tooling with Ruff

- Replace black, isort, and flake8 with Ruff (all-in-one linter and formatter)
- Add comprehensive pyproject.toml configuration
- Simplify Makefile code quality targets
- Configure exclusions for venv/.venv in pyproject.toml
- Auto-fix 1,359 linting issues across codebase

Benefits:
- Much faster builds (Ruff is written in Rust)
- Single tool replaces multiple tools
- More comprehensive rule set (UP, B, C4, SIM, PIE, RET, Q)
- All configuration centralized in pyproject.toml
- Better import sorting and formatting consistency

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

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

app/exceptions/cart.py

@@ -3,10 +3,8 @@
 Shopping cart specific exceptions.
 """
 
-from typing import Optional
 
-from .base import (BusinessLogicException, ResourceNotFoundException,
-                   ValidationException)
+from .base import BusinessLogicException, ResourceNotFoundException, ValidationException
 
 
 class CartItemNotFoundException(ResourceNotFoundException):
@@ -36,8 +34,8 @@ class CartValidationException(ValidationException):
     def __init__(
         self,
         message: str = "Cart validation failed",
-        field: Optional[str] = None,
-        details: Optional[dict] = None,
+        field: str | None = None,
+        details: dict | None = None,
     ):
         super().__init__(
             message=message,
@@ -75,7 +73,7 @@ class InvalidCartQuantityException(ValidationException):
     """Raised when cart quantity is invalid."""
 
     def __init__(
-        self, quantity: int, min_quantity: int = 1, max_quantity: Optional[int] = None
+        self, quantity: int, min_quantity: int = 1, max_quantity: int | None = None
     ):
         if quantity < min_quantity:
             message = f"Quantity must be at least {min_quantity}"