Route /v1/loras/train through queue_manager.acquire/release with a constant
"lora-train" model key. Concurrent training requests now queue and run one after
another (serialized by the scheduler, protecting the shared base cache) and
participate in the same scheduling/metrics as every other model request, instead
of being rejected with 409. _train_lock is kept as the in-flight signal that
_release_base_cache checks.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>