Implement health check endpoint on separate local port and HTTPS fallback

- Add health.rs module with start_health_check_listener() that binds to
  127.0.0.1:{health_check_port} and serves GET /health returning 200 OK
  with empty body
- Add health_route() in proxy/handler.rs for HTTPS listener fallback
- Add port conflict detection in config validation: health_check_port
  must not conflict with listener ports on 127.0.0.1/localhost/0.0.0.0
- health_check_port = 0 disables the separate listener (handled at call
  site)
- Add unit and integration tests for health check functionality

src/proxy/handler.rs

@@ -1,2 +1,12 @@
-#[allow(dead_code)]
-pub struct ProxyHandler;
+use axum::http::StatusCode;
+use axum::response::IntoResponse;
+use axum::routing::get;
+use axum::Router;
+
+async fn health_handler() -> impl IntoResponse {
+    StatusCode::OK
+}
+
+pub fn health_route() -> Router {
+    Router::new().route("/health", get(health_handler))
+}