[Supervisor Observability] Supervisor Status Tracking

[KCui0327]

📖 Description
Enable the gateway to expose supervisor status (active/inactive/failed).

✅ Objectives

• Maintain a status table of all supervisors
• Expose supervisor state through API/logs/metrics
• Ensure work is not assigned to inactive supervisors

🎯 Acceptance Criteria

• Gateway reflects accurate status of supervisors
• Status updates visible via API/logging/metrics
• Integration tests simulate multiple supervisors going online/offline