Real-time notifications via SSE

Establish a Server-Sent Events connection for real-time notifications. The server sends an `open` frame on connect, a `message` frame per notification, and a `: keepalive` comment every `notifications.sse.heartbeat_seconds` (default 25s) to keep intermediary proxies from closing the idle connection.

curl -X GET "https://example.com/v1/notifications/sse"

"event: open\ndata: {\"message\": \"Connection established\"}\n\nevent: message\ndata: {\"id\":\"...\",\"type\":\"chat\"}\n\n: keepalive\n\n"