feat(server): add Multi-Notebook Digest & Rollup Endpoint (POST /api/extended/summary/digest)

[allenporter]

Feature Request: Multi-Notebook Digest & Rollup Endpoint (POST /api/extended/summary/digest)

Summary
Add a server endpoint POST /api/extended/summary/digest to aggregate insights across multiple recently created/edited notebooks into a single daily/weekly AI rollup.

Details
Currently, summaries are generated on a per-file basis (/api/extended/file/summary/list). A global multi-notebook rollup endpoint would enable web clients and dashboard home screens to display high-level cross-notebook synthesis.

Proposed API Specification

POST /api/extended/summary/digest
Request:
{
  "days": 7
}

Response:
{
  "success": true,
  "recentNotebookCount": 5,
  "topTopics": ["SQLite Migration", "Socket.IO Sync"],
  "digestText": "Over the past 7 days, your notes focused on database schema migrations and Socket.IO connection handling..."
}