Add http_port range validation (0 or 1-65535)

Change http_port type from u16 to u32 to allow out-of-range values to be
caught by validation. Add HttpPortInvalid error variant and validation check
for http_port > 65535. Add test for http_port=65536 producing HttpPortInvalid.
http_port=0 (disabled) remains valid per existing test.

tests/integration_test.rs

@@ -259,7 +259,7 @@ async fn test_rate_limit_eviction_task() {
 
 fn make_redirect_listener_config(
     bind_addr: &str,
-    http_port: u16,
+    http_port: u32,
     https_port: u16,
 ) -> reverse_proxy::config::static_config::ListenerConfig {
     reverse_proxy::config::static_config::ListenerConfig {