feat(power): 1Hz chunked light-sleep; meter backoff; log throttling

- Replace delay() with light_sleep_chunked_ms() in sender idle path
  (100ms chunks preserve UART FIFO safety at 9600 baud)
- Add ENABLE_LIGHT_SLEEP_IDLE build flag (default: on, fallback: =0)
- Meter reader task: exponential backoff on consecutive poll failures
  (METER_FAIL_BACKOFF_BASE_MS..MAX_MS) to reduce idle Core-0 wakeups
- Configurable SENDER_DIAG_LOG_INTERVAL_MS (5s debug / 30s prod)
- Configurable METER_FRAME_TIMEOUT_CFG_MS, SENDER_CPU_MHZ
- New PlatformIO envs: lowpower, 868-lowpower, lowpower-debug
- Add docs/POWER_OPTIMIZATION.md with measurement plan and Go/No-Go

src/power_manager.cpp

@@ -9,7 +9,7 @@ static constexpr float BATTERY_DIVIDER = 2.0f;
 static constexpr float ADC_REF_V = 3.3f;
 
 void power_sender_init() {
-  setCpuFrequencyMhz(80);
+  setCpuFrequencyMhz(SENDER_CPU_MHZ);
   WiFi.mode(WIFI_OFF);
   esp_wifi_stop();
   esp_wifi_deinit();
@@ -117,6 +117,33 @@ void light_sleep_ms(uint32_t ms) {
   esp_light_sleep_start();
 }
 
+void light_sleep_chunked_ms(uint32_t total_ms, uint32_t chunk_ms) {
+  if (total_ms == 0) {
+    return;
+  }
+  if (chunk_ms == 0) {
+    chunk_ms = total_ms;
+  }
+  uint32_t start = millis();
+  for (;;) {
+    uint32_t elapsed = millis() - start;
+    if (elapsed >= total_ms) {
+      break;
+    }
+    uint32_t remaining = total_ms - elapsed;
+    uint32_t this_chunk = remaining > chunk_ms ? chunk_ms : remaining;
+    if (this_chunk < 10) {
+      // Light-sleep overhead (~1 ms save/restore) not worthwhile for tiny slices.
+      delay(this_chunk);
+      break;
+    }
+    light_sleep_ms(this_chunk);
+    // After wake the FreeRTOS scheduler runs higher-priority tasks (e.g. the
+    // meter_reader_task on Core 0) before returning here, so the UART HW FIFO
+    // is drained automatically between chunks.
+  }
+}
+
 void go_to_deep_sleep(uint32_t seconds) {
   esp_sleep_enable_timer_wakeup(static_cast<uint64_t>(seconds) * 1000000ULL);
   esp_deep_sleep_start();