review fixes: seeded-auth hard gate, ci.yml build serialization, assets 404

- M1: E2E_REQUIRE_SEEDED_AUTH=1 in the CI e2e step makes login failures hard
  failures (loginAs throws, guards disabled, globalSetup refuses a pre-populated
  DB); auth.spec redirect test asserts outright under the flag; non-admin
  /admin test is now a real authorization assertion
- M2: ci.yml build step depends_on test — never two concurrent turbo builds
  on the shared workspace
- M3: unknown /assets/* paths 404 from the SPA catch-all instead of serving
  index.html with an immutable cache header; spec arm added
- minors: e2e step gets when: *image_build_when, health poll uses
  GATEWAY_PORT + AbortSignal.timeout, BETTER_AUTH_SECRET generated per run
  (no literal in tree), failure echoes artifact path, dev-guide documents the
  gate, stale verify-release comment fixed

apps/web/e2e/admin.spec.ts

@@ -1,11 +1,14 @@
 import { test, expect } from '@playwright/test';
-import { loginAs, ADMIN_USER, TEST_USER } from './helpers/auth.js';
+import { loginAs, ADMIN_USER, REQUIRE_SEEDED_AUTH, TEST_USER } from './helpers/auth.js';
 
 test.describe('Admin page — admin user', () => {
   test.beforeEach(async ({ page }) => {
     await loginAs(page, ADMIN_USER.email, ADMIN_USER.password);
     const url = page.url();
-    test.skip(!url.includes('/chat'), 'No seeded admin user — skipping admin tests');
+    test.skip(
+      !REQUIRE_SEEDED_AUTH && !url.includes('/chat'),
+      'No seeded admin user — skipping admin tests',
+    );
   });
 
   test('admin page loads with the Admin Panel heading', async ({ page }) => {
@@ -43,26 +46,19 @@ test.describe('Admin page — non-admin user', () => {
   test.beforeEach(async ({ page }) => {
     await loginAs(page, TEST_USER.email, TEST_USER.password);
     const url = page.url();
-    test.skip(!url.includes('/chat'), 'No seeded test user — skipping non-admin tests');
+    test.skip(
+      !REQUIRE_SEEDED_AUTH && !url.includes('/chat'),
+      'No seeded test user — skipping non-admin tests',
+    );
   });
 
-  test('non-admin visiting /admin sees access denied or is redirected', async ({ page }) => {
+  test('non-admin visiting /admin never sees the admin panel', async ({ page }) => {
     await page.goto('/admin');
-    // Either redirected away or shown an access-denied message
-    const onAdmin = page.url().includes('/admin');
-    if (onAdmin) {
-      // Should show some access-denied content rather than the full admin panel
-      const hasPanel = await page
-        .getByRole('heading', { name: /admin panel/i })
-        .isVisible()
-        .catch(() => false);
-      // If heading is visible, the guard allowed access (user may have admin role in this env)
-      // — not a failure, just informational
-      if (!hasPanel) {
-        // access denied message, redirect, or guard placeholder
-        const url = page.url();
-        expect(url).toBeTruthy(); // environment-dependent — no hard assertion
-      }
-    }
+    // Wait for the app shell to render (redirect and access-denied views both
+    // keep the sidebar), then assert the panel itself is absent. globalSetup
+    // seeds TEST_USER with role 'member', so this is a real authorization
+    // assertion, not environment-dependent.
+    await expect(page.getByRole('img', { name: /mosaic logo/i })).toBeVisible({ timeout: 10_000 });
+    await expect(page.getByRole('heading', { name: /admin panel/i })).not.toBeVisible();
   });
 });