fix: apply idle timeout to Telegram relay

Wrap both sides of the Telegram relay in connIdleTimeout,
same as already done for domain fronting in #416.

Without this, if a client disappears (network drop, battery dies),
the TCP connection stays formally alive and the goroutine in the
worker pool blocks on io.CopyBuffer indefinitely. Under mass client
disconnects this accumulates zombie goroutines.

Fixes #417

mtglib/proxy.go

 	relay.Relay(
 		ctx,
 		ctx.logger.Named("relay"),
-		ctx.telegramConn,
-		ctx.clientConn,
+		connIdleTimeout{Conn: ctx.telegramConn, timeout: p.idleTimeout},
+		connIdleTimeout{Conn: ctx.clientConn, timeout: p.idleTimeout},
 	)
 }