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/meta.py

@@ -1,7 +1,8 @@
 """Meta endpoints for shared constants."""
+
 from fastapi import APIRouter
 
-from models import Permission, InviteStatus, ROLE_ADMIN, ROLE_REGULAR
+from models import ROLE_ADMIN, ROLE_REGULAR, InviteStatus, Permission
 from schemas import ConstantsResponse
 
 router = APIRouter(prefix="/api/meta", tags=["meta"])
@@ -15,4 +16,3 @@ async def get_constants() -> ConstantsResponse:
         roles=[ROLE_ADMIN, ROLE_REGULAR],
         invite_statuses=[s.value for s in InviteStatus],
     )
-