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/wwwroot/web/fis.specialties.css

@@ -0,0 +1,120 @@
+/*   basics   */
+/*  Color SCHEME */
+/*  #8396bd  */
+/*  #e3e6e6  */
+/*  #ffffff  */
+/*  #262626  */
+/*  #262626  */
+/*  #e3e6e6  */
+/* #FFC801 */
+/* #ffff4a */
+/* media breaks */
+/* other */
+/*  mixin  */
+/*   basics   */
+/*  Color SCHEME */
+/* #1b4379  #0033b3 */
+/*neu: #56a532  ; alt: rgb(32,144,119);*/
+/* 10% sw */
+.spcfrm {
+  padding: 1.5rem;
+}
+.spcfrm .spc_intro {
+  margin-bottom: 1rem;
+  padding: 0.5rem 0.6rem;
+  background: #f3f6fb;
+  border-left: 3px solid rgb(27, 67, 121);
+  font-size: 0.9rem;
+  color: #444;
+}
+.spcfrm .spc_grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(24rem, 1fr));
+  gap: 1rem;
+  align-items: start;
+}
+.spcfrm .spc_card {
+  border: 1px solid #d5d5d5;
+  border-radius: 0.4rem;
+  background: #fff;
+  box-shadow: 0 1px 3px rgba(50, 50, 50, 0.12);
+  overflow: hidden;
+}
+.spcfrm .spc_card_hd {
+  padding: 0.6rem 0.85rem;
+  background: rgb(27, 67, 121);
+  color: #fff;
+  font-weight: bold;
+}
+.spcfrm .spc_card_bd {
+  padding: 0.75rem 0.85rem;
+}
+.spcfrm .spc_card_desc {
+  font-size: 0.88rem;
+  color: #555;
+  margin-bottom: 0.75rem;
+}
+.spcfrm .spc_form .spc_field {
+  display: flex;
+  flex-direction: column;
+  gap: 0.2rem;
+  margin-bottom: 0.6rem;
+}
+.spcfrm .spc_form .spc_field label {
+  font-size: 0.85rem;
+  color: #333;
+  font-weight: 600;
+}
+.spcfrm .spc_form .spc_field input {
+  padding: 0.3rem 0.45rem;
+  border: 1px solid #bbb;
+  border-radius: 0.25rem;
+  font-size: 0.95rem;
+}
+.spcfrm .spc_form .spc_field input:invalid {
+  border-color: #c0341d;
+}
+.spcfrm .spc_form .spc_field .spc_field_note {
+  font-size: 0.78rem;
+  color: #777;
+}
+.spcfrm .spc_form .spc_actions {
+  text-align: right;
+  margin-top: 0.25rem;
+}
+.spcfrm .spc_btn {
+  cursor: pointer;
+  border-radius: 0.28rem;
+  padding: 0.3rem 0.8rem;
+  font-size: 0.9rem;
+  border: 1px solid #ababab;
+  background-color: #fff;
+  color: #333;
+}
+.spcfrm .spc_btn.spc_btn_primary {
+  background-color: rgb(86, 165, 50);
+  border-color: rgb(69.68, 133.688372093, 40.511627907);
+  color: #fff;
+}
+.spcfrm .spc_btn:disabled {
+  opacity: 0.6;
+  cursor: default;
+}
+.spcfrm .spc_result {
+  margin-top: 0.75rem;
+  padding: 0.5rem 0.6rem;
+  border-radius: 0.25rem;
+  font-size: 0.88rem;
+  word-break: break-word;
+}
+.spcfrm .spc_result .spc_result_line + .spc_result_line {
+  margin-top: 0.25rem;
+}
+.spcfrm .spc_result.spc_result_ok {
+  background: #e2f3da;
+  color: #2f6d18;
+}
+.spcfrm .spc_result.spc_result_err {
+  background: #f3ded9;
+  color: #c0341d;
+}