feat(frontend): add backfill file hashes card to admin System tab

- System tab gains a second card with a 'Datei-Hashes berechnen' button
  that calls POST /api/admin/backfill-file-hashes and shows the updated count
- i18n: admin_system_backfill_hashes_* keys added in de/en/es
- E2E: test verifies the button triggers the backfill and shows the success message

Closes #56

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

frontend/e2e/admin.spec.ts

@@ -216,3 +216,35 @@ test.describe('Admin — tag management', () => {
 		await page.screenshot({ path: 'test-results/e2e/admin-tag-restored.png' });
 	});
 });
+
+// ─── System tab — backfill file hashes ────────────────────────────────────────
+
+test.describe('Admin system tab — backfill file hashes', () => {
+	test('admin triggers file hash backfill and sees success message', async ({ request, page }) => {
+		test.setTimeout(60_000);
+
+		// Create a document via API so there is at least one without a hash
+		const createRes = await request.post('/api/documents', {
+			multipart: { title: 'E2E Backfill Hash Test' }
+		});
+		if (!createRes.ok()) throw new Error(`Create failed: ${createRes.status()}`);
+
+		await page.goto('/admin');
+		await page.waitForSelector('[data-hydrated]');
+
+		// Navigate to System tab
+		await page.getByRole('button', { name: /system/i }).click();
+
+		// Click the backfill hashes button
+		const btn = page.getByRole('button', { name: /datei-hashes berechnen/i });
+		await expect(btn).toBeVisible();
+		await btn.click();
+
+		// Success message must appear (count >= 0)
+		await expect(page.locator('text=/\\d+ Dokumente wurden aktualisiert/i')).toBeVisible({
+			timeout: 15000
+		});
+
+		await page.screenshot({ path: 'test-results/e2e/admin-backfill-hashes.png' });
+	});
+});