feat(bulk-edit): add /documents/bulk-edit route

Server load redirects READ_ALL-only users (or unauthenticated) to /documents.
Page load: onMount reads bulkSelectionStore — redirects to /documents when the
store is empty, otherwise POSTs the IDs to /api/documents/batch-metadata and
hands the resulting summaries to BulkDocumentEditLayout in mode="edit".

Refs #225

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

frontend/src/routes/documents/bulk-edit/+page.server.ts

@@ -0,0 +1,10 @@
+import { redirect } from '@sveltejs/kit';
+
+export async function load({ locals }: { locals: App.Locals }) {
+	const canWrite =
+		locals.user?.groups?.some((g: { permissions: string[] }) =>
+			g.permissions.includes('WRITE_ALL')
+		) ?? false;
+	if (!canWrite) throw redirect(303, '/documents');
+	return { canWrite };
+}