- l3-backend-3b: extend DocumentController description to include the per-month density aggregation endpoint. - l3-frontend-3b: add /documents/+page.ts (client-side gated loader) and TimelineDensityFilter component, plus relationships to the density endpoint and the search dashboard. Per Markus' follow-up §5: both diagrams are mandatory before merge. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>