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/stats.py

@@ -1,9 +1,7 @@
-import re
 from datetime import datetime
 from decimal import Decimal
-from typing import List, Optional
 
-from pydantic import BaseModel, ConfigDict, EmailStr, Field, field_validator
+from pydantic import BaseModel, Field
 
 
 class StatsResponse(BaseModel):
@@ -35,8 +33,8 @@ class CustomerStatsResponse(BaseModel):
     total_orders: int
     total_spent: Decimal
     average_order_value: Decimal
-    last_order_date: Optional[datetime]
-    first_order_date: Optional[datetime]
+    last_order_date: datetime | None
+    first_order_date: datetime | None
     lifetime_value: Decimal