feat(portal-bff): helmet + env-driven CORS allowlist + double-submit CSRF

phase-2 security baseline the main.ts placeholder has been
advertising since the auth track started. three independent
middlewares + their spa counterparts shipped together because
they only become meaningful as a set.

helmet on the bff:
  helmet() with three overrides matching our specific shape:
   - HSTS only in production (dev runs on plain http)
   - crossOriginResourcePolicy: 'cross-origin' (SPA needs to read
     bff json from a different origin)
   - CSP disabled in non-prod (bff doesn't render html; default CSP
     just adds devtools noise)

cors allowlist env-driven:
  CORS_ALLOWED_ORIGINS is now mandatory at boot. readCorsAllowlist()
  parses + validates (http(s) only, bare origins, no path/query),
  same boot-time validator family as assertSessionSecret etc. the
  previous hardcoded localhost fallback is removed — silent
  cors misconfiguration is exactly the "works in dev breaks in
  prod" trap this guard catches.

double-submit CSRF (session-bound):
  - bff mints a 256-bit csrfToken at /auth/callback, stored on
    req.session.csrfToken AND mirrored to a js-readable cookie
    (__Host-portal_csrf prod / portal_csrf dev). cookie is the
    spa's read-only view; session is the source of truth.
  - createCsrfMiddleware compares X-CSRF-Token header to the
    session token via crypto.timingSafeEqual. skips: safe methods,
    anonymous requests, /auth/login + /auth/callback.
  - mismatch → 403 {"error":"csrf"} + structured pino warn.
  - spa csrfInterceptor reads the cookie via document.cookie and
    copies into X-CSRF-Token on mutating bff requests. omitted on
    GET/HEAD/OPTIONS and on non-bff origins.
  - logout + absolute-timeout middleware now clear the csrf cookie
    alongside the session cookie.

notable choices:
  - session-bound double-submit, not pure cookie-vs-header. an
    attacker who plants a cookie via subdomain takeover would
    still need to know the server-side session token. tying the
    check to req.session.csrfToken instead of the cookie itself
    is what makes this stronger than the canonical recipe.
  - csrfInterceptor sits between bffCredentialsInterceptor (which
    sets withCredentials) and bffUnauthorizedInterceptor (which
    handles 401s). forward order, no surprises.
  - no CSRF for anonymous mutating routes in v1 — none exist today
    and generating tokens for anonymous sessions conflicts with
    express-session's saveUninitialized: false.

specs:
  - 239 / 239 pass under the clean-env repro (env -u every config
    var including the new CORS_ALLOWED_ORIGINS).
  - +42 specs across check-cors-allowlist, csrf-cookie, csrf
    middleware, csrf interceptor, and extended auth.controller /
    absolute-timeout coverage for the csrf cookie mirror/clear.

out of scope, landing in follow-ups:
  - rate limiting + structured error filter (remaining phase-2
    todos)
  - CSP fine-tuning for the portal-shell + portal-admin static
    bundles
  - csrf token rotation on idle-extension (current token lives
    the session lifetime)

apps/portal-bff/src/auth/auth.controller.spec.ts

@@ -55,6 +55,7 @@ interface SessionStub {
   user?: AuthenticatedUser;
   createdAt?: number;
   absoluteExpiresAt?: number;
+  csrfToken?: string;
   save: jest.Mock;
   destroy: jest.Mock;
 }
@@ -261,6 +262,34 @@ describe('AuthController.callback', () => {
     }
   });
 
+  it('mints a CSRF token, writes it to the session, and mirrors it to the JS-readable cookie', async () => {
+    const { controller } = makeController();
+    const res = makeResStub();
+    const session = makeSessionStub();
+    const req = makeReqStub({
+      signedCookies: { [PRE_AUTH_COOKIE_NAME]: JSON.stringify(PRE_AUTH) },
+      session,
+    });
+
+    await controller.callback(req, res, 'auth-code', PRE_AUTH.state);
+
+    // Server-side source of truth lives on the session.
+    expect(typeof session.csrfToken).toBe('string');
+    expect((session.csrfToken as string).length).toBeGreaterThan(20);
+
+    // Cookie mirror — name picked from sessionCookieName's twin
+    // (NODE_ENV-conditional). In the test default (development),
+    // it's the unprefixed variant.
+    const csrfCookieCall = res.cookie.mock.calls.find(
+      (c) => c[0] === 'portal_csrf' || c[0] === '__Host-portal_csrf',
+    );
+    expect(csrfCookieCall).toBeDefined();
+    expect(csrfCookieCall?.[1]).toBe(session.csrfToken);
+    // Must NOT be HttpOnly — the SPA reads it to echo back via the
+    // X-CSRF-Token header.
+    expect(csrfCookieCall?.[2]).toMatchObject({ httpOnly: false });
+  });
+
   it('registers the new session in the user_sessions index after save', async () => {
     const { controller, userSessionIndex } = makeController();
     const res = makeResStub();
@@ -501,6 +530,7 @@ describe('AuthController.logout', () => {
     });
     expect(session.destroy).toHaveBeenCalledTimes(1);
     expect(res.clearCookie).toHaveBeenCalledWith('portal_session', { path: '/' });
+    expect(res.clearCookie).toHaveBeenCalledWith('portal_csrf', { path: '/' });
     expect(logger.log).toHaveBeenCalledWith(
       { event: 'auth.signed_out', wasAuthenticated: true },
       'AuthLogout',