D-4.8 · deep worker health probe.

Returns 200 + healthy=true only when:

• At least one render in the last 10 minutes succeeded, OR
• The active queue is zero (nothing to do) Otherwise 200 + healthy=false with a diagnosis string. Returning a non-200 here would cause Fly to recycle the machine, which is too aggressive — we want to alert via Telegram (existing reward stall worker pattern) and keep serving. Probe-driven dashboards (uptime monitors) interpret healthy=false themselves.