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/shared_constants.py

@@ -1,4 +1,5 @@
 """Load shared constants from shared/constants.json."""
+
 import json
 from pathlib import Path
 
@@ -10,4 +11,3 @@ SLOT_DURATION_MINUTES: int = _constants["booking"]["slotDurationMinutes"]
 MIN_ADVANCE_DAYS: int = _constants["booking"]["minAdvanceDays"]
 MAX_ADVANCE_DAYS: int = _constants["booking"]["maxAdvanceDays"]
 NOTE_MAX_LENGTH: int = _constants["booking"]["noteMaxLength"]
-