feat: Implement admin-only specialties management interface

- Added MailerAttachmentPolicy for validating email attachments against server rules.
- Introduced fis.specialties.js for managing hidden maintenance actions via an admin interface.
- Created fis.specialties.scss for styling the specialties management UI.
- Added German localization for specialties management in fis.specialties_txt_de.js.
- Developed CSS for specialties management in fis.specialties.css.
- Implemented server-side stored procedure fds__admin_backupAndClearInvoiceFile for backing up and clearing invoice files.
- Created fds__invoice_file_backups table to store backup records of invoice files.

Fuchs/Docs/Concepts/admin-system-status.md

@@ -47,7 +47,7 @@ click "Administration"  → $ocms.init('admin')
   $ocms.admin.init2()   → init3() renders the page
     POST /do/admin/status → { info, probes }         full snapshot + all probes
   per-card "Aktualisieren" → POST /do/admin/probe/<component> → { probe }
-  "Test-E-Mail senden"     → POST /do/admin/testmail (to/subject/body) → { result }
+  "Test-E-Mail senden"     → POST /do/admin/testmail (to/subject/body/attachpdf) → { result }
 ```
 
 ### Backend (`ISystemStatusService` / `SystemStatusService`, scoped)
@@ -60,7 +60,7 @@ click "Administration"  → $ocms.init('admin')
   (`status` = `ok`/`error`/`disabled`/`unconfigured`, `ok`, `message`, `detail`, `durationMs`,
   optional `metrics`). They never throw — failures are captured in the result. `database` runs
   `SELECT @@SERVERNAME, DB_NAME(), SUSER_SNAME()`; `email` calls
-  `IComService.VerifyCredentialsAsync` (GET `{BaseUrl}/mailer/fc?fn=verify` with the configured
+  `IComService.VerifyCredentialsAsync` (GET `{BaseUrl}/api/mailer/verify` with the configured
   Basic-auth account/token — see the Mailer API contract) and only turns `ok` when the Mailer API
   itself confirms the credentials (`ok:true`); when `ProcessWebComSettings.ServerId` is configured
   it is sent along as `&serverid=...` so the API also confirms this server belongs to the account
@@ -74,9 +74,18 @@ click "Administration"  → $ocms.init('admin')
   count** for the invoice/reminder containers, surfaced as `metrics` and totalled in the message);
   `mfr` calls `IMfrClientFactory.Create().GetEntities()`. The generic `metrics` (label/value pairs)
   is how a probe reports extra facts for display — the blob probe uses it for the file counts.
-- `SendTestEmailAsync(to, subject, body)` HTML-encodes the body and sends via `IComService`, so
-  the `Fuchs:Email:OverrideRecipient` redirect applies exactly as for any other mail; the result
-  reports the requested recipient and, when active, the override target.
+- `SendTestEmailAsync(to, subject, body, attachPdf)` HTML-encodes the body and sends via
+  `IComService`, so the `Fuchs:Email:OverrideRecipient` redirect applies exactly as for any other
+  mail; the result reports the requested recipient and, when active, the override target.
+  With `attachPdf` (dialog select "PDF-Anhang mitsenden", default *Ja*; absent form value ⇒ on)
+  a one-page letter PDF (`Fuchs-Intranet-Testmail.pdf`) is rendered in memory via
+  `IPdfService.WriteLetterAsync` + `DocToPdfBytes` — standard Fuchs letterhead, the recipient in
+  the address window, `Datum`/`Umgebung`/`Nummer=admin_test` in the admin block, the mail's own
+  subject and body as the letter text; no DB or stored document is touched. This exercises the
+  Mailer's multipart/attachment path, so per ADR 0028 `sent` is only `true` when the Mailer also
+  confirmed the attachment (`attachmentsProcessed == 1`); the result's `attachmentName` says
+  whether an attachment was part of the send. A PDF rendering failure is reported in the result
+  (`sent=false`, message names the error) and nothing is sent.
 - Every probe emits the `fuchs.systemstatus.probes` counter tagged by component + status and an
   `systemstatus.probe` activity span.