feat: log AP signal levels and roam for a 6 dB gain

The roaming scan found two APs for the SSID every 10 minutes but the log
never showed their signal, so there was no way to tell whether the
device at -70 dBm had a better AP to move to. Log each AP on the
roaming scans, show roams at info level, and switch for a 6 dB gain
instead of 10.

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

src/ESPNetwork.h

@@ -12,6 +12,8 @@
 // changes and not more often than this.
 #define RSSI_EMIT_MIN_DBM 3
 #define RSSI_EMIT_MIN_MS 10000
+// A roaming scan only moves to another AP this much stronger.
+#define ROAM_MIN_GAIN_DB 6
 class ESPNetwork {
   protected:
     unsigned long lastEmit = 0;