Add Adaptive Polling Intervals to WebServer (#7864)

* feat: add adaptive polling intervals to WebServer

Replace fixed 5ms polling with adaptive intervals based on HTTP activity:
- 50ms during active periods (first 5 seconds after request)
- 200ms during medium activity (5-30 seconds)
- 1000ms during idle periods (30+ seconds)

Reduces CPU usage significantly during idle periods while maintaining
responsiveness when handling HTTP requests.

* Fix integer overflow and magic numbers in WebServer

- Handle millis() overflow in getAdaptiveInterval()
- Replace magic numbers with named constants
- Improve code readability and maintainability

src/mesh/http/ContentHandler.cpp

@@ -148,6 +148,8 @@ void registerHandlers(HTTPServer *insecureServer, HTTPSServer *secureServer)
 
 void handleAPIv1FromRadio(HTTPRequest *req, HTTPResponse *res)
 {
+    if (webServerThread)
+        webServerThread->markActivity();
 
     LOG_DEBUG("webAPI handleAPIv1FromRadio");
 
@@ -391,6 +393,9 @@ void handleFsDeleteStatic(HTTPRequest *req, HTTPResponse *res)
 
 void handleStatic(HTTPRequest *req, HTTPResponse *res)
 {
+    if (webServerThread)
+        webServerThread->markActivity();
+
     // Get access to the parameters
     ResourceParameters *params = req->getParams();