Add Postman collection for all API endpoints

Lets you exercise the Heartbeat, Identity, and Auth0 endpoints without
running the Python client. Covers 22 requests across auth, systems,
live, energy, weather, EMS, EV chargers, and AI optimisations.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: Markus Rebbert