Stream notifications

Establish a Server-Sent Events (SSE) stream to receive real-time notifications for the authenticated user, filtered by notification names and/or job/image IDs. Event chain on first connection (from the client's perspective):

1. Zero or more event: backfill messages, each with JSON data {id, name, data, timestamp} for previously unconsumed notifications (up to 10 minutes old), delivered oldest-first.
2. One event: backfill_complete message with data: {}. At this point the client can treat local state as synchronized.
3. Live data: messages (no explicit event field) for real-time notifications from Redis pub/sub, using the same JSON shape: {id, name, data, timestamp}.
4. Periodic :keepalive SSE comments (~every 15s) to keep the connection open through proxies/timeouts; clients should ignore them.

Consumed notifications must be deleted by the client (see DELETE /notifications/{id}), so they are not replayed on the next connection/poll.