Registers a new terminal device or re-registers an existing device for merchant onboarding. This endpoint validates the merchant's terminal payment method configuration, generates a registration code for device activation, and handles device name uniqueness validation. Returns a 6-digit activation code that expires in 24 hours for device pairing.

Key features:
Validates merchant has terminal payment method enabled.
Generates secure 6-digit registration codes.
Handles existing device re-registration with code reuse if valid.
Enforces unique device names per merchant.
Supports device status transitions (INACTIVE → REGISTERED).