Deep readiness check: database connectivity, alembic migration state, printer wiring, SNMP probe recency, print-queue liveness, SSE subscriber capacity, and stuck render slots (render_slots: page renders past LABELYARD_RENDER_TIMEOUT_S whose thread still runs; fails when every slot is stuck, a hub restart frees them). Returns 200 with status in {ready, degraded} when all critical checks pass; 503 with status=not-ready when any critical check (database / alembic) fails.