Sandboxes now survive Petri restarts. No more lost state when the server goes down.

What's new:

• SQLite-backed registry — sandbox state persists across restarts

• Sandbox TTL — idle sandboxes are automatically cleaned up after 1 hour

• TTL resets on every exec call — active sandboxes are never killed

• Configurable via PETRI_SANDBOX_TTL_SECONDS environment variable

• Cleaner exec output — dep install and execution now run separately, no more pip noise leaking through