feat: implement deep sleep with periodic wake for battery operation

## Description
Implement ESP32 deep sleep to enable battery-powered operation for remote sensor nodes.

## Requirements
- [ ] Configurable sleep interval (default 5 minutes)
- [ ] Wake -> read sensors -> report via MQTT -> sleep cycle
- [ ] Battery voltage monitoring via ADC
- [ ] Low battery alert threshold

## Skills
C/C++, ESP32, power management