Rework ListUsers to match the ListMemos pagination contract: opaque
PageToken, normalizePageSize, DB-level limit+1 look-ahead, and a
next_page_token. Adds Offset to store.FindUser with an OFFSET clause in
all three dialects, and an `id DESC` ORDER BY tiebreaker so offset pages
stay stable when created_ts ties.
Also align pagination across list endpoints:
- Bump DefaultPageSize 10 -> 50 to match the documented default; use
normalizePageSize in ListAttachments.
- Remove the never-implemented total_size field from all six list
responses (ListUsers, ListAttachments, ListMemoComments,
ListMemoReactions, ListUserSettings, ListPersonalAccessTokens) and
regenerate.
- useListUsers now pages through next_page_token so the admin members
view still loads every user past the default page size.
