M1 Beta: Define OTA update path for standalone ESP controller

[mastertape]

Roadmap reference: docs/roadmap/m1-dhl-retrofit-beta.md

Define the beta OTA update path for the standalone ESP controller.

Acceptance criteria:

• OTA mechanism candidate is selected or compared.
• Local/trusted-network update assumptions are documented.
• First successful OTA test is defined as beta acceptance criterion.
• Failure modes are listed.
• Rollback/failure-safety requirements for later versions are noted.
• OTA does not require cloud service.
• Security assumptions for update authorization are documented.