Add API Versioning / Namespacing

**Epic:** Infra

**Description**
Prepare the API for versioning by namespacing all routes under /api/v1.

**Acceptance criteria**
- All API routes live under /api/v1
- Health endpoint accessible at /api/v1/health
- Existing functionality still works

**Notes:**
Good read on the subject: https://www.honeybadger.io/blog/rails-api-versioning/