test(e2e): run the suite on one worker so a red run means something (#241)

The lever the plan deliberately held back, applied now that there is evidence it is needed. Better failure messages and a wider assertion timeout were not enough.

Measured on this branch, same commit, same machine. Two parallel runs each failed 3 of 155, and not the same three: password-reset and admin-inventory-filters in one, admin-email-settings, admin-inventory-filters and resend-verification in the next. All 16 tests from those three specs then passed when run serially, and the full suite passed 155 of 155 twice in a row. Failures that move between runs of identical code are contention, not defects — specs asserting over tables other specs are concurrently writing to.

The cost is about three minutes, roughly one minute parallel against 3.9 and 4.0 serially. A red run that means something is worth three minutes; the previous state was a suite whose result nobody could act on, which is what #245 had to skip a real test to work around.

If that time is ever needed back, the cheaper fix is giving each worker its own database rather than raising this number and reopening #241.

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

frontend/playwright.config.ts

@@ -2,7 +2,28 @@ import { defineConfig, devices } from '@playwright/test';
 
 export default defineConfig({
   testDir: './tests/e2e',
+  // Kept true so a file's tests are independent units, but with one worker it
+  // no longer interleaves them. See `workers` below.
   fullyParallel: true,
+  // One worker, because the suite shares a single database.
+  //
+  // This was the lever #241 deliberately held back, to see first whether better
+  // failure messages and a wider assertion timeout were enough. They were not.
+  // Measured on this branch, same commit, same machine: parallel runs failed 3
+  // of 155 twice over, and the three were not the same three — password-reset
+  // and admin-inventory-filters in one run, admin-email-settings,
+  // admin-inventory-filters and resend-verification in the next. Run serially,
+  // all 16 of those same tests passed; the full suite then passed 155 of 155
+  // twice in a row.
+  //
+  // So the failures were contention, not defects: specs asserting over tables
+  // that other specs were concurrently writing to. The cost is about 3 minutes
+  // — roughly 1 minute parallel against 3.9 and 4.0 serially — and a red run
+  // that means something is worth three minutes.
+  //
+  // The cheaper fix, if that time is ever needed back, is giving each worker
+  // its own database rather than raising this number and reopening #241.
+  workers: 1,
   retries: process.env.CI ? 1 : 0,
   reporter: [['list']],
   use: {