Backend - Multi-Tenant Onboarding Transactional API

[inteee]

Context: Creating a new clinic involves creating the Clinic record AND the first Admin user. This must happen in a single database transaction to prevent orphaned data.
Location: apps/api/src/modules/clinics/onboarding.controller.ts
Requirements:

1. Create POST /clinics/register.
2. Accept payload: Clinic Name, Admin Email, Admin Password.
3. Use a Mongoose Transaction (Session) to insert the Clinic, retrieve its _id, and insert the User with role CLINIC_ADMIN tied to that clinicId.
   Acceptance Criteria:

• If user creation fails, the clinic creation rolls back (database transaction works).
• Registration returns a valid JWT to auto-login the new admin.