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/sum.py

@@ -1,13 +1,13 @@
 """Sum calculation routes."""
+
 from fastapi import APIRouter, Depends
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from auth import require_permission
 from database import get_db
-from models import User, SumRecord, Permission
+from models import Permission, SumRecord, User
 from schemas import SumRequest, SumResponse
 
-
 router = APIRouter(prefix="/api/sum", tags=["sum"])