Release process automation and maturity

### Context

The 0.42.0 release process (#678) exposed several manual steps and gaps in our release tooling. While the current `scripts/apache_release.py` handles the basics (build, sign, SVN upload, vote template), there is no automation for RC promotion, release branches, backports, reproducible builds, or release notes.

Looking at how other Apache projects (especially Airflow) handle releases, there are clear improvements we can adopt to make the process more reliable and less error-prone.

### Current state

- `scripts/apache_release.py` handles build, GPG signing, SHA-512 checksums, and SVN upload
- `scripts/verify_apache_artifacts.py` handles voter verification
- Vote email template is generated but sent manually
- RC promotion from `dist/dev` to `dist/release` is fully manual
- No release branches, no backport automation, no reproducible builds

### Goals

1. Make cutting an RC a single-command operation
2. Make promoting a passed RC to final release a single-command operation
3. Introduce release branches for safe cherry-picking during vote periods
4. Automate release notes generation
5. Ensure builds are reproducible (bit-for-bit identical from same source)

### Sub-issues

- [ ] #716 Reproducible builds with `SOURCE_DATE_EPOCH`
- [ ] #717 RC promotion script (SVN dev -> release)
- [ ] #718 Release branch model (`vX.Y-test` + `vX.Y-stable`)
- [ ] #721 Auto-backport workflow for release branches
- [ ] #723 Release notes automation
- [ ] #720 Voter verification improvements
- [ ] #719 Vote and announcement email tooling
- [ ] #722 Milestone management workflow
- [ ] #747 Release CI follow-ups: tighter coverage