Add authentication, Redis pub/sub, and error handling to SSE

Improvements:
- Add Redis pub/sub support for multi-server deployments
- Add authentication check to SSE stream endpoint
- Add 30s heartbeat keepalive for long-lived connections
- Implement exponential backoff reconnection logic in client
- Fix TypeScript types (websiteId optional, timer types)
- Use specific query key invalidation instead of broad match
- Fix undefined access in session-events listener map

src/app/api/send/route.ts

@@ -152,7 +152,7 @@ export async function POST(request: Request) {
         distinctId: id,
         createdAt,
       });
-      emitSessionCreated(sourceId, sessionId);
+      await emitSessionCreated(sourceId, sessionId);
     }
 
     // Visit info