fix(airflow): a login that survives a container restart

The simple auth manager generates a random password on first start and
writes it to a file, so every restart of the api-server invalidated the
last one and the password had to be dug out of the container logs again.

The stack now writes that file itself from AIRFLOW_ADMIN_PASSWORD before
exec'ing the api-server. Airflow generates nothing when the file already
exists, so the login is whatever the stack environment says it is.

Written with python rather than echo, so json.dumps escapes a password
containing quotes, backslashes or non-ASCII correctly — verified against
`p@ss "wo\rd' £5`, which round-trips intact.

An unset AIRFLOW_ADMIN_PASSWORD raises KeyError and the container exits.
Falling back to a generated password would silently undo the point of the
change, and a compose-level `:?` gives the same refusal a readable reason.
This does mean the variable MUST be set in Portainer before the next
deploy of either stack.

Not affected by the two Docker gotchas in the upstream docs: this image
has no USER directive so it runs as root, and the file is rewritten from
the environment on every start rather than persisted on a volume.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BvdDKvFFSZuMVDH5fEyTob

docs/DEPLOY.md

@@ -98,6 +98,12 @@ fail the E2E gate. That's the point: staging absorbs the risk.
    pr-checks status checks (frontend, backend, builds, ai-review) to pass.
 5. **Bootstrap staging data via Airflow** (no prod dump — staging populates
    itself from source, exercising the pipeline image end-to-end):
+   - Set `AIRFLOW_ADMIN_PASSWORD` in the stack environment first. The
+     api-server refuses to start without it. Airflow's simple auth manager
+     otherwise generates a password on first start and writes it to a file, so
+     the login changes every time the container restarts; the stack writes that
+     file itself from this variable instead. `AIRFLOW_ADMIN_USER` defaults to
+     `admin`.
    - Open the staging Airflow UI (`http://<host>:8081`) and trigger, in order:
      `school_data_daily`, `school_data_monthly_ofsted`, then the manual-schedule
      `school_data_annual_ees` and `school_data_annual_idaci`.