SSE stream of speak-start / speak-end events, scoped to the caller.

H4: this was an UNAUTHENTICATED global event bus — every tenant's generation_id / profile_name / client_id was fanned out to every listener, so an anonymous subscriber watched all tenants' activity live. On the public engine an anonymous caller (no valid token) is now refused with a 404, and an authenticated caller sees only events tagged with its own tenant. SaaS-off → tenant is None and the desktop stream is byte-identical.