OCR ran outside the shared GPU governors: /v1/ocr(/batch) and run_ocr_step
call OcrManager.ocr_document directly, bypassing request_model, so OCR
neither waited on thermal cooldown nor evicted/released VRAM against
LLM/diffusion models.

- thermal: ocr_document now awaits thermal.wait_until_safe (via to_thread)
  before OCR-ing pages, so OCR honours the same cooldown/front-pause gate as
  every other GPU workload.
- VRAM (both directions):
  * evict-before-build: when a pool loads its instances, ask the model
    manager to free size*per-instance-VRAM first (_evict_models_for_vram),
    so OCR contends on equal footing instead of OOMing.
  * releaser: OcrManager registers _release_vram(needed_gb) via
    register_external_vram_releaser, tearing down built pools so loading an
    LLM/diffusion model can reclaim OCR VRAM. Pools rebuild lazily next request.
- engine cleanup() (SYNC, eviction-thread safe) + vram_gb() footprint per
  engine: docTR ~0.7 GB (frees model + empty_cache), subprocess ~2.5 GB
  (kills worker). _Pool tracks all instances and release_sync() tears them down.
- configure() now tears down stale pools on reconfigure (prompt VRAM free
  instead of GC-deferred).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mw2KQiswmD69T45fTfjKwW