Add daily digest notifications

[chindris-mihai-alexandru]

Goal\nSend a daily summary with top 5 prioritized emails.\n\n## Requirements\n- Configurable schedule (default 08:00 local time)\n- Uses same scoring rules as realtime alerts\n- Delivery via selected notifier (matrix/ntfy/slack/etc.)\n- Includes brief reasons for ranking\n\n## Notes\n- Consider simple cron/interval (node-cron) for MVP\n- Store last digest time to avoid duplicates\n