Make signaling WebSockets reconnectable in place

reconnect(freshToken) replaces only the socket to the signaling service.
The signaling instance, its handlers, and everything built on it (server
channel, WebRTC factories, live peer connections) survive, so a
signaling drop no longer requires tearing the transport down.

Liveness is now detectable on idle servers: a WebSocket protocol ping
every 15 seconds guarantees inbound pongs on a healthy socket, so
isChannelAlive(maxSilence) also catches silently half open TCP.

Per channel scheduled tasks are tracked and cancelled on channel
inactive, so reconnects no longer leak ping loops. TURN credential
pushes are applied for the lifetime of the socket instead of only
during connect, and the JSON RPC endpoint refreshes credentials every
30 minutes, so late joining peers no longer receive expired relay
credentials. Pending RPC requests fail fast when the socket dies. The
frame aggregator limit is raised to 128 KB for batched RPC frames.