Export a full JSON backup with all account metadata and snapshots.
Includes all account fields (type, institution, color, is_liability,
category, sort_order, archived_at) and nested snapshots (date, balance,
source, note). No date range filter — exports everything.
Optional sections query param (comma-separated section names)
restricts the export to only those sections. Omitting it exports
everything (today’s behavior, unchanged).