fix: apply idle timeout to domain fronting relay connections

Domain fronting relay (for non-Telegram traffic) had no idle timeout,
causing worker pool exhaustion under traffic spikes.

The ProxyOpts.IdleTimeout field existed but was never wired into the
proxy. Now domain fronting connections are wrapped with per-read/write
deadlines reset to the configured idle timeout (default 1m), so stale
or slowloris-style connections are reaped promptly.

Fixes #378

internal/cli/run_proxy.go

@@ -262,6 +262,7 @@ func runProxy(conf *config.Config, version string) error { //nolint: funlen
 
 		AllowFallbackOnUnknownDC: conf.AllowFallbackOnUnknownDC.Get(false),
 		TolerateTimeSkewness:     conf.TolerateTimeSkewness.Value,
+		IdleTimeout:              conf.Network.Timeout.Idle.Get(mtglib.DefaultIdleTimeout),
 
 		DoppelGangerURLs:    doppelGangerURLs,
 		DoppelGangerPerRaid: conf.Defense.Doppelganger.Repeats.Get(mtglib.DoppelGangerPerRaid),