Decide MQTT topic ownership between firmware defaults and Radxa config

[TalkingJupiter]

Context

The firmware currently generates a telemetry topic locally, while RuntimeConfig also includes telemetry/status/event topic fields. This creates an ownership question: are topics firmware-defined or Radxa-provisioned?

Decision needed

Choose one model:

1. Firmware always publishes to generated topics like repacss/devices/<mac>/telemetry.
2. Firmware uses generated topics until Radxa config arrives, then switches to configured topics.
3. Radxa config is required before telemetry is considered configured.

Things to consider

• Unknown/unconfigured devices should probably still be able to send hello and maybe basic telemetry.
• Configured topics allow flexible rack mapping and future migrations.
• Generated topics are simpler and easier to debug.

Acceptance criteria

• Document the chosen topic ownership model.
• Update firmware implementation to match the decision.
• Update host/MQTT docs so broker subscribers know which topics are authoritative.