fix(e2e): run Playwright single-worker to stop OOM page-crash in verify-stack (refs #115)

verify-stack intermittently failed on the e2e with "Page crashed" mid-action:
Playwright defaulted to 2 workers, so two full chromium browsers ran alongside the
whole compose stack on the 8 GB runner and the renderer was OOM-killed. Pin
workers: 1 (only two long-running happy-path specs) and add --disable-dev-shm-usage,
removing the memory contention rather than masking it with retries (CLAUDE.md §15).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/e2e/playwright.config.ts

@@ -12,6 +12,12 @@ export default defineConfig({
   timeout: 90_000,
   expect: { timeout: 15_000 },
   retries: 1,
+  // Run the specs serially. Each spec drives a full `channel: 'chromium'` browser, and the e2e
+  // shares an 8 GB runner with the entire compose stack (OpenZaak, NRC, Keycloak, Flowable, 4×
+  // Postgres, every service + 3 portals). Two parallel browsers exhaust memory and the renderer is
+  // OOM-killed mid-action ("Page crashed") — fixing the flakiness at its source rather than leaning
+  // on `retries` (CLAUDE.md §15). Only two long-running happy-path specs, so serial costs little.
+  workers: 1,
   reporter: [['list']],
   use: {
     baseURL,
@@ -26,7 +32,13 @@ export default defineConfig({
     // headless), not Playwright's default headless-shell, so pin `channel: 'chromium'`.
     channel: 'chromium',
     launchOptions: {
-      args: [`--unsafely-treat-insecure-origin-as-secure=${baseURL},${behandelURL}`],
+      args: [
+        `--unsafely-treat-insecure-origin-as-secure=${baseURL},${behandelURL}`,
+        // Write Chromium's shared memory to /tmp instead of the container's small /dev/shm, so a
+        // large DOM/heap can't crash the renderer on the memory-constrained runner (belt-and-braces
+        // alongside the single worker above).
+        '--disable-dev-shm-usage',
+      ],
     },
   },
   projects: [{ name: 'chromium', use: { ...devices['Desktop Chrome'] } }],