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-shell/src/app/app.config.ts

@@ -7,8 +7,10 @@ import { provideHttpClient, withFetch, withInterceptors } from '@angular/common/
 import { provideRouter } from '@angular/router';
 import {
   AUTH_BFF_BASE_URL,
+  AUTH_CSRF_COOKIE_NAME,
   bffCredentialsInterceptor,
   bffUnauthorizedInterceptor,
+  csrfInterceptor,
 } from 'feature-auth';
 import { environment } from '../environments/environment';
 import { appRoutes } from './app.routes';
@@ -33,13 +35,18 @@ export const appConfig: ApplicationConfig = {
     //     when a BFF route (other than `/auth/me` itself) answers
     //     401, keeping the SPA's auth state in sync after server-
     //     side session destruction (absolute-timeout, manual revoke).
+    //   - `csrfInterceptor` reads the BFF's CSRF cookie and copies
+    //     the value into the `X-CSRF-Token` header on mutating BFF
+    //     requests (POST / PUT / PATCH / DELETE), per the
+    //     double-submit pattern from ADR-0009.
     provideHttpClient(
       withFetch(),
-      withInterceptors([bffCredentialsInterceptor, bffUnauthorizedInterceptor]),
+      withInterceptors([bffCredentialsInterceptor, csrfInterceptor, bffUnauthorizedInterceptor]),
     ),
-    // `feature-auth` is environment-agnostic and reads the BFF base
-    // URL from this token — provided once per app from `environment.ts`
-    // (per ADR-0018).
+    // `feature-auth` is environment-agnostic and reads its BFF
+    // config from these tokens — provided once per app from
+    // `environment.ts` (per ADR-0018).
     { provide: AUTH_BFF_BASE_URL, useValue: environment.bffApiBaseUrl },
+    { provide: AUTH_CSRF_COOKIE_NAME, useValue: environment.bffCsrfCookieName },
   ],
 };

apps/portal-shell/src/environments/environment.ts

@@ -25,6 +25,15 @@ export const environment = {
    */
   bffApiBaseUrl: 'http://localhost:3000/api',
 
+  /**
+   * Name of the BFF's CSRF cookie. Mirrors the BFF's
+   * `csrfCookieName()` convention: `__Host-portal_csrf` in
+   * production (Secure-required, hence excluded from HTTP dev),
+   * `portal_csrf` otherwise. Read by the `csrfInterceptor` to
+   * echo the value back in the `X-CSRF-Token` request header.
+   */
+  bffCsrfCookieName: 'portal_csrf',
+
   /**
    * OTLP/HTTP traces endpoint. Targets the Collector in
    * `infra/local/dev.compose.yml`. CORS is enabled there — see