* feat(models): add per-alias concurrency admission
Add registry-backed FIFO admission limits with queue-aware deadlines and full-stream leases. Expose max_concurrency through storage, admin configuration, OpenAPI, documentation, and diagrams, with role and live backend count coverage.
* fix(api): omit null concurrency schema default
Keep max_concurrency optional for presence-keyed updates without advertising a null default for its non-null integer OpenAPI shape.
Patrick Buckley