Add import content manifests and review UI
Generate content_manifest.jsonl in all six file importers and thread import_id through the FileImporter protocol so manifests land in the canonical imports/<timestamp> directory.
Add map_items_to_segments() for segment mapping, lazy manifest backfill via generate_content_manifest(), and two new import content API endpoints for paginated/filterable content lists and item detail.
Extend the import detail view with a Content tab, search debounce, month filter, expand-on-click detail loading, and #content hash activation. Add a Browse what was imported CTA to the workspace completion summary.
Add fixture imports plus unit/API coverage for manifest writing, backfill, pagination/filtering, detail lookup, missing-import handling, and importer import_id threading. Update affected API baselines for the new fixture corpus.