[Backend] Add a `/health` endpoint

[Muhammadcodes112]

Description

A minimal endpoint deploy platforms (and humans) can hit to confirm the server is actually running.

Where

astre_server/src/app.controller.ts

What to implement

@Get('health')
getHealth() {
  return { status: 'ok', timestamp: new Date().toISOString() };
}

Acceptance Criteria

• GET /health returns 200 with { status: 'ok', timestamp: <ISO string> }
• No authentication required

Hints

• This is one of the simplest issues in the whole backend list — a good very-first-PR pick.

[sidd51]

I'd like to work on this, please assign it to me.