Firmware: accept enabled:false to support remote disable

[TalkingJupiter]

Context

docs/mqtt-contract.md §4 defines enabled: false as a valid, applied config that means "provisioned but stay silent" — the remote on/off lever (e.g. to quiet a device during an OTA). The current firmware does the opposite: RuntimeConfig::isValid() returns false when !enabled, so a disabled config is rejected and never applied. The device therefore keeps its previous enabled: true state and cannot be turned off remotely — the dead end noted earlier in this work.

Conflict to resolve

This directly contradicts open issue #6, which lists enabled among fields that must be required/validated (i.e. keeps rejecting enabled: false). The contract supersedes that assumption. #6 should be updated or closed as part of this — its premise (require topic fields + enabled in config) no longer matches the design.

Scope

Make enabled: false an accepted, applied config; keep telemetry gated so a disabled device stays silent.

Implementation notes

• isValid() must stop treating !enabled as invalid (it still requires configured, mac, rack_id, role).
• The telemetry send path already gates on config_.enabled (two separate guards in sendMQTT), so once a disabled config is applied, transmission stops correctly — the only change is letting it be applied.
• Confirm the hello loop stays gated on !configured only, so a disabled-but-configured device does not resume announcing.

Acceptance criteria

• A config with enabled: false is accepted and applied; the device stops publishing telemetry but remains connected and subscribed.
• A later enabled: true config re-enables transmission.
• Matches docs/mqtt-contract.md §4 and §10.

Relates to #6.