feat: Enhance invoice file backup procedure to include JSON snapshots

- Updated the stored procedure `fds__admin_backupAndClearInvoiceFile.sql` to back up not only the invoice file but also JSON snapshots of related data (invoices, items, details, service requests) to ensure consistency and traceability.
- Modified the table structure of `fds__invoice_file_backups` to accommodate new JSON columns for storing these snapshots.
- Added an ADR (0030) document to outline the decision and rationale behind these changes, emphasizing the importance of maintaining a consistent set of data alongside the backed-up file.

Fuchs/js/intranet/modules/fis.specialties.js

@@ -1,4 +1,5 @@
-/* "Specialties" — admin-only page (URL /specialties, menu Administration → Spezialfunktionen)
+/* "Specialties" — admin-only page (URL /specialties, menu Administration → Spezialfunktionen;
+   the menu entry is only visible when the dropdown is opened with Shift held, see fis_main_menu.js)
    that exposes the otherwise hidden maintenance actions as cards with small forms. Each card
    calls its real action endpoint (which enforces the fds_sys > 4 gate itself); this module only
    renders and reports. Add a card per action in `cards`. */