No idle timeout on established gate tunnels

open   by a73x

Labels: backlog

From the pre-launch security audit (finding GATE-4, ranked #5 of the outstanding set).

Established gate tunnels have no idle timeout. The per-connection handshake defense is
already good (30s handshakeGrace), and concurrent unauthenticated handshakes are now
bounded (MaxStartups, shipped). What remains is that an AUTHENTICATED tenant can hold
idle tunnels open indefinitely, which compounds the connection-exhaustion surface rather
than standing alone.

**Fix.** An idle deadline on the established tunnel. Low; authenticated-tenant only.