Keep receiver timesync fast and extend sender fast window

- Receiver now sends time sync every 60s indefinitely (mains powered)
- Sender stays in fast timesync listen mode for first 60s even with RTC

README.md

@@ -123,7 +123,7 @@ void receiver_loop() {
   }
 
   if (time_to_send_timesync()) {
-    time_send_timesync(self_short_id); // 60s for first 10 min, then hourly
+    time_send_timesync(self_short_id); // always every 60s (receiver is mains-powered)
   }
 
   mqtt_loop();
@@ -299,8 +299,10 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
 - Defaults: `pool.ntp.org` and `time.nist.gov`.
 ## RTC (DS3231)
 - Optional DS3231 on the I2C bus. Connect SDA to GPIO21 and SCL to GPIO22 (same bus as the OLED).
+- Enable/disable with `ENABLE_DS3231` in `include/config.h`.
 - Receiver time sync packets set the RTC.
 - On boot, if no LoRa time sync has arrived yet, the sender uses the RTC time as the initial `ts_utc`.
+- Receiver keeps sending time sync every 60 seconds.
 
 ## Build Environments
 - `lilygo-t3-v1-6-1`: production build (debug on)