fix(e2e): run Playwright single-worker to stop OOM page-crash in verify-stack (closes #115) (#116)

## What & why

`verify-stack` was failing intermittently on the Playwright e2e with `Page crashed` mid-action (`locator.fill`) and 90s timeouts — the run logged **"2 workers"**, i.e. two full `channel: 'chromium'` browsers running alongside the entire compose stack on the 8 GB self-hosted runner. The renderer gets OOM-killed. Tests passed only when a retry happened to run alone.

Fix: pin `workers: 1` in `tests/e2e/playwright.config.ts` (there are only two long-running happy-path specs, so serial costs little) and add `--disable-dev-shm-usage`. This removes the memory contention at the source rather than leaning on `retries` (CLAUDE.md §15 — flaky tests are fixed, not retried).

Closes #115

## Definition of Done

- [x] Linked Gitea issue (#115).
- [ ] Failing test committed first — N/A: the "red" is the observed `verify-stack` e2e crash (`Page crashed`, 2 workers); this changes test-harness config to fix it. Verified green by re-running the e2e (see notes).
- [x] Conventional Commit referencing the issue (`refs #115`).
- [ ] CI green — the point of the change; `verify-stack` e2e should stop OOM-crashing.
- [x] Docs — none needed (test-config only; rationale in an inline comment).
- [ ] ADR — N/A.

## Notes for reviewers

- One-line-of-behaviour change: `workers: 1` + `--disable-dev-shm-usage`; no product or spec changes.
- `Page crashed` is a renderer OOM, not a product defect — the happy path passes when a browser runs alone (the flaky retries already showed this). Single-worker makes that the normal case.
- Independent of #110 (that PR fixes `docker-compose.local.yml`; this fixes the CI `verify-stack` e2e). Landing this first unblocks #110's `verify-stack`.

Reviewed-on: #116

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'] } }],