feat(admin): enhance adult verification file handling and localization

- Added a new method in AdminService to resolve adult verification file paths, improving file retrieval logic.
- Updated the AdminVerificationView to display a message when the verification document is missing.
- Localized the missing document message in German, English, and Spanish for better user experience.

frontend/src/i18n/locales/en/admin.json

@@ -45,6 +45,7 @@
             "previewTitle": "Proof preview",
             "closePreview": "Close preview",
             "previewUnavailable": "No inline preview is available for this file type.",
+            "documentMissing": "The verification file was not found on the server.",
             "empty": "No matching requests found.",
             "loadError": "Could not load approvals.",
             "updateError": "Could not update the status.",