Embedding admission gate:
- default min-interval 200ms -> 300ms (paces GPU starts harder)
- expose embed_max_concurrency / embed_max_backlog / embed_min_interval_ms
  per-model in the admin model editor (embedding-gated section), round-tripped
  through /admin/api/model-configure into models.json

Engine supervisor circuit breaker:
- quarantine an engine that exits crashloop_max (5) times within
  crashloop_window (120s) instead of respawning it ~1/s forever. A GPU that has
  fallen off the bus (Polaris secondary-bus-reset bug) makes its engine die on
  every launch; the breaker takes radeon cleanly out of routing (embeddings fail
  fast) instead of hammering a dead card. Manual restart_engine() clears it.

Pairs with host-side RX580 mitigations (dpm=high lock, lockup_timeout=25s,
conc=1/300ms on the 3 vulkan embedding models, reset watchdog + thermal sampler).