chore: prep 2.1.0a1 alpha release with procrastinate support

[snopoke]

Summary

• Bumps version to 2.1.0a1 so the procrastinate integration (Add Procrastinate integration #46) can ship as a PEP 440 alpha for early users. PyPI installs only with `--pre`, leaving the next stable 2.1.0 uncommitted.
• Drops `--format "{base}"` from the dunamai call in `release.yml`. `{base}` strips the pre-release suffix and broke two things:
  • the tag/version equality check (`v2.1.0a1` != `v2.1.0`)
  • the artifact-upload path — built wheels are named `taskbadger-2.1.0a1-`, but the workflow looked for `taskbadger-2.1.0-`

Default dunamai output is already PEP 440, so the fix is general: beta/rc tags will also work going forward.

Release flow after this merges

1. Merge this PR to `main`.
2. Tag: `git tag v2.1.0a1 && git push origin v2.1.0a1`.
3. The `release.yml` workflow builds, runs tests, drafts a GitHub release.
4. Publishing the draft release triggers `publish.yml` → PyPI (uploads as a pre-release; `pip install taskbadger` still gets 2.0.0 unless users pass `--pre`).

Test plan

• `uv build` locally produces `taskbadger-2.1.0a1-py3-none-any.whl` and `.tar.gz`
• CI passes on this PR
• After tagging, verify the release workflow's version-check step passes (tag `v2.1.0a1` == `PROJECT_VERSION` `2.1.0a1`)
• After publishing the draft release, confirm PyPI shows 2.1.0a1 as a pre-release and `pip install --pre taskbadger` resolves to it