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.
- Updated memo and reaction filtering logic to use a unified engine for compiling filter expressions into SQL statements.
- Removed redundant filter parsing and conversion code from ListMemoRelations, ListReactions, and ListAttachments methods.
- Introduced IDList and UIDList fields in FindMemo and FindReaction structs to support filtering by multiple IDs.
- Removed old filter test files for reactions and attachments, as the filtering logic has been centralized.
- Updated tests for memo filtering to reflect the new SQL statement compilation approach.
- Ensured that unsupported user filters return an error in ListUsers method.
