- Add services/startup_health.py with health check functions:
- get_startup_status(): Returns current startup status
- readiness_under_auth_context(): Validates tenant DB under auth context
- run_startup_health_routine(): Runs all startup health checks
- Add /health/readiness endpoint for tenant DB validation
- Update startup_event() to use run_startup_health_routine()
- Add raise to startup_event to fail fast on errors
