fix: do not fail the release when pyproject.toml is already current (#18)

## The defect

The release step added in #17 commits the computed version unconditionally:

```sh
git add pyproject.toml
git commit -m "chore(release): v${VERSION}"
```

If `pyproject.toml` already carries that version — a re-run of a release, or a version corrected by hand — nothing is staged, `git commit` exits non-zero, and `set -euo pipefail` fails the job. By that point the image has already been pushed, so the release is half-done: published container, no tag. The next run then computes the same version again from the same commits.

## The fix

Skip the commit and its branch push when there is nothing staged; tag either way.

## Verification

The step was extracted from the workflow and run against a scratch repository with a real `origin`:

- `VERSION` equal to the file's version → logs `pyproject.toml is already at 0.1.0`, pushes the tag, no commit.
- `VERSION` different → commits `chore(release): v0.2.0`, pushes the branch, then the tag.

Both paths leave `main` and the tags consistent. `bash -n` clean.

Found while porting this same release scheme to `music-mirror`, which carries the guarded version from the start.

---------

Co-authored-by: Emma Thorpe <emma.thorpe@citrix.com>
Reviewed-on: #18

.gitea/workflows/build-and-publish.yaml

@@ -188,11 +188,19 @@ jobs:
           git config user.name "${{ github.actor }}"
           git config user.email "${{ github.actor }}@users.noreply.${REGISTRY}"
           git add pyproject.toml
-          git commit -m "chore(release): v${VERSION}"
 
+          # The file may already carry this version, in which case there is
+          # nothing to commit and `git commit` would fail the job after the
+          # image has already been pushed.
+          if git diff --cached --quiet; then
+            echo "pyproject.toml is already at ${VERSION}"
+          else
+            git commit -m "chore(release): v${VERSION}"
             # Push the branch before the tag. If main has moved on and this push
             # is rejected, the job fails without having left a tag pointing at a
             # commit that is not on main.
             git push origin "HEAD:${GITHUB_REF_NAME}"
+          fi
+
           git tag -a "v${VERSION}" -m "v${VERSION}"
           git push origin "v${VERSION}"