test: revert workers to 1 to avoid login concurrency

All login-based tests share MOICEN_E2E_UNIONID and cannot run in
parallel without interfering with each other's session state.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

playwright.config.ts

@@ -8,7 +8,7 @@ export default defineConfig({
   testDir: './tests',
   testIgnore: ['**/admin_debug.spec.ts'],
   timeout: 120_000,
-  workers: 2,
+  workers: 1,
   expect: { timeout: 30_000 },
   use: {
     baseURL: