TickerAll: Subscribe to restart events (broadcast on planned restarts)

Add a way for clients to be notified when TickerAll performs a planned restart or maintenance, so automated systems can react gracefully instead of only seeing their connection drop. Today, when a service is brought down for a deploy or restart, live broker sessions and streams drop and clients reconnect blind. Instead, let a client subscribe to a maintenance/restart event — delivered over the existing WebSocket stream and/or an outbound webhook. When TickerAll is about to bring a service down, broadcast the event to all subscribers, ideally with a short lead time and an estimated back-online window, so bots can pause new orders, optionally flatten, and prepare to reconnect. A follow-up all-clear event can fire once the service is back. Why: automated trading systems need a clean, deterministic signal to handle maintenance windows, rather than treating a planned restart the same as an unexpected disconnect. To scope: the event schema (type, scheduled time, reason, estimated duration), delivery channels (WebSocket event vs outbound webhook), lead time, and whether an all-clear event fires on recovery.