test(e2e): replace unchecked lookups, widen the assertion timeout, restore the skipped test (#241)

Tasks 2 to 4 of the plan. Nine `collection.find(...)` dereferences become findOrFail, so a missing row fails as a named assertion naming what was wanted and how many rows were searched, rather than "Cannot read properties of undefined" pointing at test plumbing. Where the old code followed the lookup with expect(x).toBeTruthy(), that assertion is dropped: findOrFail already guarantees it, and with a better message.

The expect timeout goes from Playwright's default 5s to 10s. It costs nothing on a green run — it bounds how long a failing assertion waits, not how long a passing one takes — and #239 died reporting exactly Timeout: 5000ms on a runner that also builds, migrates and runs three other suites.

The admin-save happy path comes back from the #245 skip. It is the only end-to-end check that adding an item reaches the database rather than merely firing a toast, and it passed both full parallel runs and in isolation.

filters.spec.ts:216 is deliberately untouched: its .find() searches CSS class names on a string array, not test data, and has no missing-row failure mode.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/tests/e2e/admin-inline-category.spec.ts

@@ -1,4 +1,4 @@
-import { test, expect, uniqueSuffix, createCategory } from './fixtures';
+import { test, expect, uniqueSuffix, createCategory, findOrFail } from './fixtures';
 
 test.describe('Inline category creation from the item form', () => {
   test('creates a category without leaving the item form and assigns it', async ({
@@ -32,8 +32,13 @@ test.describe('Inline category creation from the item form', () => {
     await expect(page.getByText('Item added')).toBeVisible();
 
     const items = await (await page.request.get('/api/admin/items')).json();
-    const saved = items.find((item: { name: string }) => item.name === itemName);
-    expect(saved).toBeTruthy();
+    // findOrFail already fails with the row count when the item is missing,
+    // which the toBeTruthy assertion this replaces could not report.
+    const saved = findOrFail(
+      items,
+      (item: { name: string }) => item.name === itemName,
+      `the item named ${itemName}`
+    );
     expect(saved.category_name).toBe(categoryName);
   });