fix: eliminate all compiler warnings

- Remove unused imports across multiple modules
- Add #![allow(dead_code)] for public API items preserved for future use
- Add #![allow(deprecated)] for rand::Rng::gen_range usage
- Add #![allow(unused_assignments)] in main.rs
- Add #![allow(unreachable_code)] in network/stun.rs
- Prefix unused variables with underscore (_ip_tracker, _prefer_ipv6)
- Fix unused_must_use warning in tls_front/cache.rs

This ensures clean compilation without warnings while preserving
public API items that may be used in the future.

src/metrics.rs

@@ -2,7 +2,7 @@ use std::convert::Infallible;
 use std::net::SocketAddr;
 use std::sync::Arc;
 
-use http_body_util::{Full, BodyExt};
+use http_body_util::Full;
 use hyper::body::Bytes;
 use hyper::server::conn::http1;
 use hyper::service::service_fn;