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/transport/middle_proxy/health.rs

@@ -14,6 +14,7 @@ use super::MePool;
 
 const HEALTH_INTERVAL_SECS: u64 = 1;
 const JITTER_FRAC_NUM: u64 = 2; // jitter up to 50% of backoff
+#[allow(dead_code)]
 const MAX_CONCURRENT_PER_DC_DEFAULT: usize = 1;
 
 pub async fn me_health_monitor(pool: Arc<MePool>, rng: Arc<SecureRandom>, _min_connections: usize) {