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/helpers/auth.ts

@@ -12,17 +12,29 @@ export const ADMIN_USER = {
   name: 'E2E Admin User',
 };
 
+/**
+ * Set when the database was seeded by global-setup (CI sets it in the
+ * publish.yml e2e step). Seeded credentials MUST work, so login failures are
+ * hard failures and the skip-when-login-fails guards are disabled — otherwise
+ * a login regression would skip every authenticated suite and the gate would
+ * pass while proving nothing. Unset (a live environment used as a test
+ * target), the guards stay on and unseeded credentials skip their suites.
+ */
+export const REQUIRE_SEEDED_AUTH = process.env['E2E_REQUIRE_SEEDED_AUTH'] === '1';
+
 /**
  * Fill the login form and submit, then wait for the post-login redirect to
- * /chat. On failed login the wait times out and is swallowed: the page stays
- * on /login, and the callers' `test.skip(!url.includes('/chat'))` guards see
- * that. Without this wait, every guard read page.url() before the redirect
- * happened and skipped its suite even when login succeeded (#1445).
+ * /chat. Under REQUIRE_SEEDED_AUTH a missed redirect throws (failing the
+ * test). Otherwise the timeout is swallowed: the page stays on /login and the
+ * callers' `test.skip(...)` guards see that. Without this wait, every guard
+ * read page.url() before the redirect happened and skipped its suite even
+ * when login succeeded (#1445).
  */
 export async function loginAs(page: Page, email: string, password: string): Promise<void> {
   await page.goto('/login');
   await page.getByLabel('Email').fill(email);
   await page.getByLabel('Password').fill(password);
   await page.getByRole('button', { name: /sign in/i }).click();
-  await page.waitForURL(/\/chat/, { timeout: 10_000 }).catch(() => {});
+  const redirect = page.waitForURL(/\/chat/, { timeout: 10_000 });
+  await (REQUIRE_SEEDED_AUTH ? redirect : redirect.catch(() => {}));
 }