fix: health checks no longer modify endpoint updated_at timestamp

[IonesioJunior]

Summary

• Health monitor status updates now preserve the endpoint's updated_at timestamp
• Previously, every health check cycle triggered SQLAlchemy's onupdate hook, causing all health-checked endpoints to have their updated_at refreshed
• This caused endpoints from users with many endpoints (e.g., cambridge-press-oa with 177) to dominate public listings sorted by updated_at DESC
• Fix: Explicitly set updated_at=EndpointModel.updated_at in the atomic UPDATE statement, bypassing the ORM's onupdate hook

Test plan

• Verify health checks run successfully without errors
• Confirm endpoints from various users appear in public listings (not just users with many endpoints)
• Check that updated_at only changes when users actually update their endpoints