Helm chart: support bidirectional Airflow metadata DB reconciliation on helm upgrade (downgrade as well as upgrade)

### Description Today the official Airflow Helm chart's `migrateDatabaseJob` only runs forward `airflow db migrate`. Doing a `helm upgrade` that targets an **older** `airflowVersion` than the one currently running leaves the metadata DB schema ahead of the running image, and the api-server pod fails to start. The chart should reconcile the DB schema in **both** directions — upgrade *and* downgrade — based on the dispatched `airflowVersion`. ### Use case/motivation We operate Airflow on Kubernetes via this chart and ship to multiple environments through CI. We need rollback to be a first-class operation: - Deploying an older release tag (`helm upgrade` with an older `airflowVersion`) should bring the cluster — schema included — back to that release. - Today this requires out-of-band tooling: detecting current vs target version, then `kubectl exec`ing into the still-running old api-server pod and invoking `airflow db downgrade --to-version <target> --yes` **before** helm starts rolling the new image. We've implemented this as a workflow step driven by a small bash script, but it duplicates logic for every team using the chart and only helps people deploying via GitHub Actions — not ArgoCD, not manual helm. A chart-native solution would mean: set `airflowVersion: <older>` in values, run `helm upgrade`, the chart reconciles the schema, the new (older) pods come up. #### Hard constraint that shapes the design `airflow db downgrade --to-version X.Y.Z` requires the alembic revision scripts for **every revision between the current head and the target**. Those scripts only ship inside the image of the version that **introduced** them. So: | Direction | Image that must run the operation | Why | |---|---|---| | Upgrade (current < target) | **Target** image | Forward revisions ship in the target image | | Downgrade (current > target) | **Currently running** image | Reverse-direction code for revisions to undo only exists in the current image | | Same | none | No-op | Today's `m…