Global stream endpoint for Datastar live sessions.
Handles GET /__datastar/stream?token=... requests. Verifies the stream token
(using Phoenix.Token-based signing), subscribes to the session's GenServer, and
enters the SSE loop (PhoenixDatastar.Server.enter_loop/2).
Returns 401 if the token is missing or invalid. Returns 409 if the GenServer is no longer running (e.g., session expired), signaling the client to reload.
Expected route
get "/__datastar/stream", PhoenixDatastar.StreamPlug, :stream