Cover the admin API added in 03cb4c7 for duplicate-content decisions:
- GET /api/admin/hash-conflicts — pending conflict groups with member
items and per-item usage counts (progress, highlights, bookmarks,
notes, collections)
- POST /api/admin/hash-conflicts/:id/resolve — action=keep (merge child
rows into keep_uuid, delete losers) vs action=keep_all (dismiss);
JSON and form-encoded bodies, error codes including 409 for already
resolved
- When conflicts are created (startup backfill, rescans) and the
guarantee that files on disk are never deleted
