[BE-84] Introduce API versioning (/api/v1)

[yusuftomilola]

Overview

Routes are unversioned. With a public frontend and a Vercel deployment already live, any breaking change to a response shape will break clients with no migration path.

Tasks

• Enable app.enableVersioning({ type: VersioningType.URI, defaultVersion: '1' }) in main.ts.
• Ensure the global prefix yields /api/v1/....
• Update frontend/lib/apiClient.ts API_BASE_URL and .env.example accordingly.
• Keep unversioned paths working via a temporary redirect or alias for one release, then remove.
• Document the versioning and deprecation policy in the README.

Acceptance Criteria

• All endpoints respond under /api/v1.
• The frontend works against the versioned base URL with no other change.
• Swagger reflects the versioned paths.

Notes for Contributors

Comment below to be assigned.