Signal-Based Idle Hibernation for MCPX Instances

MCPX instances that sit idle now hibernate automatically, freeing up resources with no action needed — the next request wakes them right back up.

The admin dashboard now surfaces two distinct signals so it's easy to tell what's idle versus active:

• Last Seen — the instance is up and connected.
• Last Used — someone actually made a request.

Configurable via HIBERNATION_IDLE_MINUTES, with sensible safeguards built in to prevent accidental fleet-wide hibernation.