List Learning Users

List the users that own learning records, with a per-user count and last-updated timestamp. Intended as the entry point for a per-user view: list users here, then drill into a single user's learnings via GET /learnings?user_id=.... Records with no owner (user_id IS NULL) are excluded. Pass learning_type to restrict the grouping to a single store (e.g. user_profile or user_memory). For a scoped (non-admin) caller results are bound to that user; an explicit user_id that differs is rejected with 403. Admins and unscoped callers list all users. Sortable by user_id or last_learning_updated_at (the default).