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>

ml/experiments/bench/mlflow_client.py

@@ -10,8 +10,7 @@ Why not the official ``mlflow`` SDK? Two reasons specific to the oO setup:
    Pulling a 200MB SDK transitively for that is excess weight.
 
 All calls are synchronous httpx with explicit ``Host`` so the script can
-run from the host shell, from inside docker, or from Airflow workers
-without further config.
+run from the host shell or from inside docker without further config.
 """
 
 from __future__ import annotations