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/INVOICE_LIFECYCLE.md

@@ -216,7 +216,9 @@ flowchart TD
 ## 6. Sending the invoice by email
 
 Email delivery is handled by `IComService` / `ProcessWebComService`, which
-talks to the **ProcessWeb Mailer API** (`push_com`, `comType=email`).
+talks to the **ProcessWeb Mailer API** (`POST /api/mailer/push_com`,
+`comType="e"`; contract: https://processweb.de/api/mailer/fc, client side
+described in `Docs/Concepts/processweb-mailer-interface.md`).
 
 - `HandleRequestSconf` builds the HTML body via `BuildInvoiceBody(balance,
   paymentTerms)` — a fixed German thank-you text, the amount (if non-zero),
@@ -234,8 +236,12 @@ talks to the **ProcessWeb Mailer API** (`push_com`, `comType=email`).
   - A signature is appended to the HTML body.
   - If the mailer is disabled (`_settings.Enabled == false`), sending is a
 	documented no-op.
-  - Attachments are base64-encoded inline (`{ filename, mimeType,
-	contentBase64 }`) and POSTed to the Mailer API.
+  - Attachments are pre-validated against the Mailer's limits (size, count,
+	no empty file, extension rules) and POSTed as `multipart/form-data` —
+	envelope JSON in an unnamed field plus one file part per attachment, with
+	`attachmentCount` in the envelope.
+  - The Mailer's 200 response must confirm `attachmentsProcessed` equal to
+	the number sent; otherwise the send is reported as failed (ADR 0028).
 - Only on a **successful** send does the controller call
   `fds__setInvoiceSent @auto=true`, which sets `IsSent = 1` — so a failed
   mailer call leaves the invoice finalised but *not marked sent* (visible via
@@ -252,8 +258,10 @@ sequenceDiagram
 	C->>Com: SendEmailAsync("inv_<Id>", subject, body, email, "", {pdf})
 	Com->>Com: validate email, apply OverrideRecipient (dev), append signature
 	alt mailer enabled
-		Com->>API: POST push_com {comType:"email", attachments:[base64 pdf]}
-		API-->>Com: success/failure
+		Com->>Com: MailerAttachmentPolicy.Validate(pdf)
+		Com->>API: POST /api/mailer/push_com (multipart: envelope{comType:"e", attachmentCount:1} + pdf part)
+		API-->>Com: 200 {comStackID, attachmentsProcessed} / 4xx/5xx
+		Com->>Com: attachmentsProcessed == 1 ? success : failure
 	else mailer disabled
 		Com-->>Com: no-op (logged)
 	end