S9: Power autonomy for harbor-deployed sensors

[xishell]

Persona

Harbor maintenance

User outcome

As harbor maintenance, I want sensors to run autonomously on solar + battery, so that I am not visiting them constantly to swap batteries.

A sensor that drains its battery in days is not deployable. Solar charging + low-power sleep make the network self-sufficient.

Acceptance Criteria

• Sensor runs on a single 18650 cell with solar top-up
• Battery level reported via ADC voltage divider
• Low-power sleep between sensor reads / publishes
• Battery life > 7 days under typical conditions, > 3 days with no sun
• Battery level visible in MQTT diagnostic payload