Add rate limiting to login endpoint

Security:
- Add slowapi dependency for rate limiting
- Create rate_limit.py module with configurable limits
- Apply 5 requests/minute limit to login endpoint
- Make rate limit configurable via ORCHARD_LOGIN_RATE_LIMIT env var

Testing:
- Set high rate limit (1000/min) in docker-compose.local.yml for tests
- All 265 tests pass

backend/requirements.txt

@@ -10,6 +10,7 @@ pydantic-settings==2.1.0
 python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
 bcrypt==4.0.1
+slowapi==0.1.9
 
 # Test dependencies
 pytest>=7.4.0