Add API versioning with consistent route structure

[Wilfred007]

What

Routes exist at both /api/ and /api/v1/. The apiVersionMiddleware exists but versioning is not consistently applied. Some routes are duplicated.

Why

Breaking changes will affect existing integrations. Inconsistent versioning makes API evolution impossible.

Scope

In scope: All routes under /api/v1/, /api/ as alias with deprecation header
Out of scope: v2 routes

Acceptance Criteria

• All routes accessible under /api/v1/
• /api/ routes return deprecation warning header
• Version in response header
• No breaking changes to v1

Technical Context

• backend/src/middlewares/apiVersionMiddleware.ts — versioning middleware
• backend/src/routes/v1/ — v1 routes
• backend/src/app.ts — route mounting (both /api/ and /api/v1/)