Get frontend build version

[Kvanzi]

As a user of the system I want to see which frontend build is bundled into the running application so that backend and frontend versions can be checked in one place.

Assumptions & Constraints

1. The frontend is built from a separate repository inside the backend CI pipeline and packaged into the same jar, so its commit data can only be captured at that moment.
2. A local backend build has no frontend repository available, therefore frontend fields will be empty outside CI.
3. The image is rebuilt only when a backend build runs - on push to the dev branch or on a manual workflow run from the deploy branch - so frontend version data changes only together with the next backend build.
4. Depends on the backend version story (Get backend build version #547): the endpoint must already exist.

Preconditions

1. The version endpoint is implemented.
2. The CI pipeline checks out the frontend repository before the jar is built.

Business rules

1. The returned values must describe the exact frontend build bundled into the artifact, not the latest commit of the frontend repository.
2. Absence of frontend data is a valid state and must be represented by empty values, not by an error.
3. Adding frontend data must not change the existing backend fields of the response.

Acceptance Criteria

1. GET /api/v1/version returns the frontend commit hash and commit date grouped under a frontend object.
2. The returned values match the frontend build that is packaged into the running artifact.
3. When the artifact was built without frontend data, the fields are empty and the endpoint still returns 200.
4. A local build without frontend data completes successfully.
5. Backend fields of the response remain unchanged.
6. All dates are returned in ISO-8601 format.
7. The new fields are documented in Swagger.

Out of scope

1. Any UI work.

Epic link
Epic #546

Labels to be added
"User story", "Versioning", "Application Versioning"

Tasks

1. Add frontend build version to version endpoint #551