chore: remove Airflow completely from the stack

Drop all four Airflow containers (db, init, webserver, scheduler) from the
mlops compose profile, leaving MLflow as the sole mlops service. Remove
AIRFLOW_* env vars, config fields, health-check entries, DAG trigger code
in admin/bench routes, the airflow_dag_run_id schema column, Airflow nav
links and DAG-run links in the admin UI, the two Airflow DAG files
(bench_dag.py, sim_dag.py), and all related docs/ADR references.
Simulations now run exclusively via the subprocess path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/architecture/overview.md

@@ -47,7 +47,6 @@ User reactions (done / snooze / dismiss) are events too. They close the loop as
 - **OpenAPI** for HTTP; TS client auto-generated; Python pydantic hand-written while consumers are few.
 - **Feast** for feature store when we get there; homegrown adapter until then (Phase 1 seam).
 - **MLflow** for model registry and experiment tracking; deployed at `o.alogins.net/mlflow`.
-- **Airflow** for batch pipelines; deployed at `o.alogins.net/airflow`.
 - **Auth.js** embedded behind an OIDC-shaped boundary (ADR-0004). Swap to a standalone OIDC provider when mobile ships.
 - **k3s** as the first step beyond docker-compose — no "compose → full k8s" cliff.