Code References: Add vcs_provider support and GitLab CI template

Context: https://github.com/Flagsmith/flagsmith/issues/7000
Depends on: https://github.com/Flagsmith/flagsmith/issues/7134

The upload script doesn't send `vcs_provider` in the API payload, and there's no GitLab CI template. The scanning logic itself is already platform-agnostic.

### Scope

Make the upload script send `vcs_provider`, and create a GitLab CI template that reuses the existing Python scanning logic.

### Upload script

`src/code_references/cli/upload_code_references.py`: read `$VCS_PROVIDER` (default `"github"`), include it in the JSON body. Add `vcs_provider: NotRequired[str]` to the `FeatureFlagCodeReferencesScan` TypedDict in `types.py`.

Wire it through the GitHub Actions too: add a `vcs_provider` input to `.github/actions/upload-code-references/action.yml` (default `"github"`) and forward it in the reusable workflow. Existing GitHub users are unaffected.

### GitLab CI template

New file: `templates/gitlab-ci.yml`

Consumed via [`include:remote`](https://docs.gitlab.com/ci/yaml/#includeremote) from the raw GitHub URL. Use [`spec:inputs`](https://docs.gitlab.com/ci/yaml/#specinputs) with `$[[ inputs.* ]]` interpolation so users can pass `flagsmith_project_id` and optionally `flagsmith_admin_api_url`.

Design points:
- GitLab shared runners use Docker executors, so the template must specify an [`image`](https://docs.gitlab.com/ci/yaml/#image) — `ghcr.io/astral-sh/uv:python3.14-alpine` has `uv` and Python pre-installed.
- Run in the [`.post` stage](https://docs.gitlab.com/ci/yaml/#stages) so it doesn't interfere with the project's own pipeline.
- Only run on the default branch: `rules: - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH` (both are [predefined variables](https://docs.gitlab.com/ci/variables/predefined_variables.html)).
- Map [`$CI_PROJECT_URL`](https://docs.gitlab.com/ci/variables/predefined_variables.html) to `REPOSITORY_URL` and [`$CI_COMMIT_SHA`](https://docs.gitlab.com/ci/variables/predefined_variables.html) to `REVISION`.
- Hardcode `VCS_PROVIDER=gitlab`.
- The API key comes from `$FLAGSMITH_CODE_REFERENCES_API_KEY`, a [CI/CD variable](https://docs.gitlab.com/ci/variables/#define-a-cicd-variable-in-the-ui) the user sets in project settings.
- Run all three steps (fetch, scan, upload) in a single job via `uv run --from git+https://github.com/Flagsmith/ci.git@v1.0.0`.

### Tests

- Upload with explicit `vcs_provider` includes it in the payload
- Upload without `vcs_provider` defaults to `"github"`
- `main()` reads `$VCS_PROVIDER` from the environment

### Acceptance criteria

- [ ] Existing GitHub CI clients unaffected
- [ ] `templates/gitlab-ci.yml` works with [`include:remote`](https://docs.gitlab.com/ci/yaml/#includeremote) + [`inputs:`](https://docs.gitlab.com/ci/yaml/#includeinputs)
- [ ] Template reuses the same Python scanning logic as GitHub Actions
- [ ] All new code has test coverage