Add ESP32-C3 sensor, MQTT, and OTA firmware path

[keilogic]

Summary

• add an ESP32-C3 PlatformIO target while keeping the existing ESP32 dev target
• implement analog + DHT sensor reads with compact serial/MQTT status payloads
• add MQTT reconnect/backoff and ArduinoOTA handling with credential-free defaults
• document compile-time configuration and hardware validation assumptions

Validation

• python -m platformio run -e esp32c3
• python -m platformio run -e esp32
• git diff --check

Refs #13