fix: send wifiStrength only for real signal changes

At -70 dBm the RSSI moves a dBm or two constantly, so the >1 dBm check
every 1.5s sent wifiStrength to every client about 270 times an hour.
That is extra traffic on an already lossy link and a state write in
Home Assistant for each one.

Broadcast on a channel change, or for a change of at least 3 dBm no
more than every 10s. A newly connected client still gets the current
value straight away.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Utils.h

@@ -56,6 +56,10 @@ struct rebootDelay_t {
   else if(str[0] == 'f' || str[0] == 'F' || str[0] == '0') return false;
   return def;
 }
+// Whether a wifiStrength broadcast is worth sending: always on a channel change,
+// otherwise only for an RSSI change of at least minDelta dBm and not more often
+// than every minMs.
+bool rssiNeedsEmit(int rssi, int lastRssi, int channel, int lastChannel, uint32_t sinceLastMs, int minDelta, uint32_t minMs);
 
 class Timestamp {
   char _timeBuffer[128];