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.spec.ts

@@ -0,0 +1,46 @@
+import { describe, expect, it } from 'vitest';
+import { load } from './+page.server';
+
+describe('/documents/bulk-edit +page.server.ts', () => {
+	it('redirects to /documents when user lacks WRITE_ALL', async () => {
+		const locals = { user: { groups: [{ permissions: ['READ_ALL'] }] } };
+		try {
+			// @ts-expect-error — partial event shape sufficient for this guard
+			await load({ locals });
+			throw new Error('expected redirect to be thrown');
+		} catch (e) {
+			const err = e as { status?: number; location?: string };
+			expect(err.status).toBe(303);
+			expect(err.location).toBe('/documents');
+		}
+	});
+
+	it('redirects when user has no groups', async () => {
+		const locals = { user: { groups: [] } };
+		try {
+			// @ts-expect-error — partial event shape sufficient for this guard
+			await load({ locals });
+			throw new Error('expected redirect');
+		} catch (e) {
+			expect((e as { status?: number }).status).toBe(303);
+		}
+	});
+
+	it('redirects when no user is logged in', async () => {
+		const locals = {};
+		try {
+			// @ts-expect-error — partial event shape sufficient for this guard
+			await load({ locals });
+			throw new Error('expected redirect');
+		} catch (e) {
+			expect((e as { status?: number }).status).toBe(303);
+		}
+	});
+
+	it('returns canWrite=true for a WRITE_ALL user', async () => {
+		const locals = { user: { groups: [{ permissions: ['WRITE_ALL', 'READ_ALL'] }] } };
+		// @ts-expect-error — partial event shape sufficient for this guard
+		const result = await load({ locals });
+		expect(result).toEqual({ canWrite: true });
+	});
+});