feat: expose hcsr04.enabled + last reading in /discovery

Adds an hcsr04 block to /discovery so the HA integration can decide
whether to create the distance sensor entity without probing.

Companion change: shailensobhee/ESPSomfy-RTS-ha PR #1.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: shailensobhee

src/Web.cpp

@@ -456,6 +456,10 @@ void Web::handleDiscovery(AsyncWebServerRequest *request) {
     resp.beginArray("groups");
     serializeGroups(resp);
     resp.endArray();
+    resp.beginObject("hcsr04");
+    resp.addElem("enabled", settings.HCSR04.enabled);
+    resp.addElem("distanceCm", hcsr04.lastDistanceCm);
+    resp.endObject();
     resp.endObject();
     resp.endResponse();
     net.needsBroadcast = true;