Adds SSE-based push notifications so changes made on one device
appear instantly on all other connected devices for the same user.
- SSE notification hub (src/lib/sse.ts) with per-user client tracking
- Stream endpoint (/api/events/stream) with 30s heartbeat
- Task and list API routes notify connected clients on mutations
- Client auto-connects with 5s reconnect on connection loss
- Existing 2-minute polling sync remains as fallback
v1.53.0
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
