Add API versioning convention

[Subhransu-De]

What to build

Add a simple API versioning convention so future projects built from this template have a stable path for breaking changes.

Acceptance criteria

• Routes are grouped under a documented version prefix such as /api/v1 or an equivalent convention.
• Health endpoints remain outside versioned business API routes if that is the chosen convention.
• OpenAPI docs show the versioned API clearly.
• Tests are updated for the route convention.
• README explains how future versions should be introduced.

Blocked by

None - can start immediately.