- Add get_available_slots() and _expand_availability_to_slots() to ExchangeService
- Update routes/exchange.py to use ExchangeService.get_available_slots()
- Remove all business logic from get_available_slots endpoint
- Add AvailabilityRepository to ExchangeService dependencies
- Add Availability and BookableSlot imports to ExchangeService
- Fix import path for validate_date_in_range (use date_validation module)
- Remove unused user_repo variable and import from routes/invites.py
- Fix mypy error in ValidationError by adding proper type annotation
- 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