Add ruff linter/formatter for Python

- Add ruff as dev dependency
- Configure ruff in pyproject.toml with strict 88-char line limit
- Ignore B008 (FastAPI Depends pattern is standard)
- Allow longer lines in tests for readability
- Fix all lint issues in source files
- Add Makefile targets: lint-backend, format-backend, fix-backend

backend/routes/counter.py

@@ -1,12 +1,12 @@
 """Counter routes."""
+
 from fastapi import APIRouter, Depends
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from auth import require_permission
 from database import get_db
-from models import Counter, User, CounterRecord, Permission
-
+from models import Counter, CounterRecord, Permission, User
 
 router = APIRouter(prefix="/api/counter", tags=["counter"])