M1 Beta: Define MQTT events and local API foundation

[mastertape]

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

Define the beta MQTT and local API surface for standalone operation and future bridge integration.

Acceptance criteria:

• MQTT publishes opening events.
• MQTT publishes credential/input events without raw secrets.
• MQTT publishes door status and basic controller status.
• MQTT remote open command is specified as security-sensitive and logged.
• Local API foundation is sketched for status readout.
• Future external configuration is considered but does not need to be complete in beta.
• Home Assistant integration remains optional via MQTT/API.
• No vendor lock-in is introduced.