feat(test): M5.3 — Playwright e2e harness for the dev stack

Adds the M5.3 deliverable scoped to local-dev (stage doesn't exist yet,
so the CI e2e job is gated behind the repo variable RUN_E2E='true' —
defaults off).

Layout:
  playwright.config.ts   chromium project; baseURL defaults to
                         http://acme.localhost:3000 (subdomain routing
                         fires). PLAYWRIGHT_BASE_URL / APEX_URL / etc.
                         env vars override for stage.
  tests/e2e/apex.spec.ts        landing page renders
  tests/e2e/tenant.spec.ts      signed-out dashboard shows Sign in
                                button; unknown slug returns 404
  tests/e2e/health.spec.ts      every dev-stack endpoint reachable
                                (portal /api/auth/providers, tenant-
                                registry /healthz, KC realm metadata)

Run locally with the full dev stack up:

  cd platform/orca-platform && make dev-up
  cd platform/tenant-registry && make dev
  cd platform/portal && make dev
  cd platform/portal && make e2e

OIDC click-through not asserted yet — Keycloak in headless mode is
flaky and depends on a stable test-user password. The current gate
(Sign-in button visible) catches the more common 'auth completely
broken' regression; the deeper smoke lands when stage has its own
test fixture.

tsconfig now excludes tests/e2e so vitest + tsc don't fight over
Playwright type imports.

Refs: M5.3

package.json

@@ -14,7 +14,9 @@
     "start": "next start --port 3000",
     "lint": "eslint . --max-warnings 0",
     "typecheck": "tsc --noEmit",
-    "test": "vitest run --coverage"
+    "test": "vitest run --coverage",
+    "e2e": "playwright test",
+    "e2e:install": "playwright install --with-deps chromium"
   },
   "dependencies": {
     "next": "16.2.6",
@@ -23,6 +25,7 @@
     "react-dom": "19.0.0"
   },
   "devDependencies": {
+    "@playwright/test": "^1.60.0",
     "@types/node": "20.16.10",
     "@types/react": "19.0.1",
     "@types/react-dom": "19.0.1",