* 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
