Reduce sender power draw (RX windows + CPU/WiFi/ADC/pins)

- Add LoRa idle/sleep/receive-window helpers and use short RX windows for ACK/time sync

- Schedule sender time-sync windows (fast/slow) and track RX vs sleep time in debug

- Lower sender power (80 MHz CPU, WiFi/BT off, reduced ADC sampling, unused pins pulldown)

- Make SERIAL_DEBUG_MODE a build flag, add prod envs with debug off, and document changes

src/power_manager.cpp

@@ -10,7 +10,10 @@ static constexpr float BATTERY_CAL = 1.0f;
 static constexpr float ADC_REF_V = 3.3f;
 
 void power_sender_init() {
+  setCpuFrequencyMhz(80);
+  WiFi.mode(WIFI_OFF);
   esp_wifi_stop();
+  esp_wifi_deinit();
   btStop();
   analogReadResolution(12);
   pinMode(PIN_BAT_ADC, INPUT);
@@ -22,14 +25,19 @@ void power_receiver_init() {
   pinMode(PIN_BAT_ADC, INPUT);
 }
 
-void read_battery(MeterData &data) {
-  const int samples = 8;
-  uint32_t sum = 0;
-  for (int i = 0; i < samples; ++i) {
-    sum += analogRead(PIN_BAT_ADC);
-    delay(5);
+void power_configure_unused_pins_sender() {
+  // Board-specific: only touch pins that are known unused and safe on TTGO LoRa32 v1.6.1
+  const uint8_t pins[] = {32, 33};
+  for (uint8_t pin : pins) {
+    pinMode(pin, INPUT_PULLDOWN);
   }
-  float avg = static_cast<float>(sum) / samples;
+}
+
+void read_battery(MeterData &data) {
+  uint32_t sum = 0;
+  sum += analogRead(PIN_BAT_ADC);
+  sum += analogRead(PIN_BAT_ADC);
+  float avg = static_cast<float>(sum) / 2.0f;
   float v = (avg / 4095.0f) * ADC_REF_V * BATTERY_DIVIDER * BATTERY_CAL;
 
   data.battery_voltage_v = v;