Stream real-time notifications via SSE

Server-Sent Events (SSE) endpoint for real-time notification delivery. Connect with EventSource to receive push notifications instantly.

Event types:

• connected — Sent once on connection with { "unread_count": N }
• notification — Full notification object pushed in real-time
• : heartbeat — SSE comment sent every 30s to keep the connection alive

Example (JavaScript):

const es = new EventSource('/api/v1/notifications/stream', {
  headers: { 'Authorization': 'Bearer <jwt>' }
});
es.addEventListener('connected', (e) => {
  const { unread_count } = JSON.parse(e.data);
});
es.addEventListener('notification', (e) => {
  const notification = JSON.parse(e.data);
  // Show toast, update badge, etc.
});

Architecture: Uses the cache Pub/Sub for instant delivery with automatic fallback to DB polling (5s interval) if the cache is unavailable.