Search for collections

Finds collections whose name (last path segment) or description contains the query as a case-insensitive substring. Prefer q so / and % stay in the query string; GET /collections/search/{query} remains for the web UI. Name matching is limited to the last path segment. Description matching is a substring (e.g. europ matches European). Both queries use a slim projection. hasChild is computed at read time from direct children. When limit is set, Mongo sorts by last path segment and applies skip/limit; the response is { collections, total, offset, limit, has_more, archived_included }. Without limit the response stays a full array for the web UI. archived=all is silently treated as false when the caller cannot view archives, and archived_included on the page wrapper reports the effective inclusion rather than the requested one. archived=true without archive access fails. Results are limited to collections the caller can access. Reads from MongoDB.