Backpressure and connection management

A few operational notes:

• Disable buffering at every hop (the framework, and reverse proxies via the appropriate header), or events will be batched and the "real-time" feel disappears.
• Heartbeats (periodic comments) keep idle connections alive through proxies that time out quiet streams.
• Clean up server-side resources when the client disconnects mid-stream (abort the model call if possible) so an abandoned turn doesn't keep burning tokens.