feat(documents): add delete button to document edit form

- DELETE /api/documents/{id} endpoint (204 No Content, WRITE_ALL required)
- DocumentService.deleteDocument() — throws 404 if not found, cascades
  via DB foreign keys (versions, annotations, comments all ON DELETE CASCADE)
- Delete form action in edit page server: redirects to / on success
- Two-step confirmation in the save bar: first click reveals inline
  "Wirklich löschen?" + confirm/cancel, avoiding native browser dialogs
- i18n key doc_delete_confirm added to de/en/es

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/en.json

@@ -24,6 +24,7 @@
 	"btn_edit": "Edit",
 	"btn_create": "Create",
 	"btn_delete": "Delete",
+	"doc_delete_confirm": "Really delete this document? This action cannot be undone.",
 	"btn_back_to_overview": "Back to overview",
 	"btn_back": "Back",
 	"btn_back_to_document": "Back to document",