Library: Limit default document list to 20 most recently edited

Problem

The library currently fetches ALL documents on load. This worked when we had 30-40 documents, but now with 1000+ documents:

• Slow initial page load
• Excessive API payload size
• Contributes to rate limit issues on page load

Solution

Change the default library query to return only the last 20 most recently edited documents.

API Changes

• GET /api/all should default to limit=20 and sort=updatedAt:desc
• Add pagination support if not already present
• Add "Load more" or infinite scroll in UI

Acceptance Criteria

• [ ] Library loads in <1 second with 1000+ documents in DB
• [ ] Default view shows 20 most recently edited
• [ ] User can load more documents on demand
• [ ] Search still works across all documents

Context

Currently hitting rate limits and slow loads because the frontend fires many parallel requests on page load, and /api/all returns the entire document collection.