ci: auto-backport workflow for release branches

[andreahlert]

Part of #715

Depends on the release branch model being adopted first.

Problem

Once release branches exist, cherry-picking fixes from `main` to `vX.Y-test` is a manual process that is easy to forget or do incorrectly.

Solution

Add a GitHub Actions workflow that auto-backports PRs to release branches when a specific label is applied.

How it works

1. Maintainer merges a PR to `main`
2. Maintainer adds label `backport-to-vX.Y` (e.g. `backport-to-v0.42`)
3. Workflow triggers, cherry-picks the merge commit to `vX.Y-test`
4. If clean, opens a PR to `vX.Y-test` automatically
5. If conflicts, comments on the original PR asking for manual cherry-pick

References

• Airflow's `automatic-backport.yml`
• tibdex/backport action

Acceptance criteria

• Workflow triggers on label addition
• Creates backport PR automatically on clean cherry-pick
• Comments on original PR if conflicts arise
• Labels to be created: `backport-to-vX.Y` (one per active release series)

[andreahlert]

@skrawcz @elijahbenizzy this depends on #718 (release branches) being adopted first. if we go with the branch model, having auto-backport via labels would save a lot of manual cherry-picking. just backport-to-v0.43 on a merged PR and it opens a backport PR automatically. worth it or premature?