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/src/routes/documents/[id]/edit/+page.server.ts

@@ -58,5 +58,20 @@ export const actions = {
 		}
 
 		throw redirect(303, `/documents/${params.id}`);
+	},
+
+	delete: async ({ params, fetch }) => {
+		const baseUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+
+		const res = await fetch(`${baseUrl}/api/documents/${params.id}`, {
+			method: 'DELETE'
+		});
+
+		if (!res.ok) {
+			const backendError = await parseBackendError(res);
+			return fail(res.status, { error: getErrorMessage(backendError?.code) });
+		}
+
+		throw redirect(303, '/');
 	}
 };