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>

models/schema/cart.py

@@ -3,8 +3,6 @@
 Pydantic schemas for shopping cart operations.
 """
 
-from datetime import datetime
-from typing import List, Optional
 
 from pydantic import BaseModel, ConfigDict, Field
 
@@ -43,7 +41,7 @@ class CartItemResponse(BaseModel):
     line_total: float = Field(
         ..., description="Total price for this line (price * quantity)"
     )
-    image_url: Optional[str] = Field(None, description="Product image URL")
+    image_url: str | None = Field(None, description="Product image URL")
 
 
 class CartResponse(BaseModel):
@@ -51,7 +49,7 @@ class CartResponse(BaseModel):
 
     vendor_id: int = Field(..., description="Vendor ID")
     session_id: str = Field(..., description="Shopping session ID")
-    items: List[CartItemResponse] = Field(
+    items: list[CartItemResponse] = Field(
         default_factory=list, description="Cart items"
     )
     subtotal: float = Field(..., description="Subtotal of all items")
@@ -82,7 +80,7 @@ class CartOperationResponse(BaseModel):
 
     message: str = Field(..., description="Operation result message")
     product_id: int = Field(..., description="Product ID affected")
-    quantity: Optional[int] = Field(
+    quantity: int | None = Field(
         None, description="New quantity (for add/update operations)"
     )