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)

pnpm-lock.yaml

@@ -126,6 +126,9 @@ importers:
       express-session:
         specifier: ^1.19.0
         version: 1.19.0
+      helmet:
+        specifier: ^8.1.0
+        version: 8.1.0
       ioredis:
         specifier: ^5.10.1
         version: 5.10.1
@@ -6510,6 +6513,10 @@ packages:
     resolution: {integrity: sha512-F/1DnUGPopORZi0ni+CvrCgHQ5FyEAHRLSApuYWMmrbSwoN2Mn/7k+Gl38gJnR7yyDZk6WLXwiGod1JOWNDKGw==}
     hasBin: true
 
+  helmet@8.1.0:
+    resolution: {integrity: sha512-jOiHyAZsmnr8LqoPGmCjYAaiuWwjAPLgY8ZX2XrmHawt99/u1y6RgrZMTeoPfpUbV96HOalYgz1qzkRbw54Pmg==}
+    engines: {node: '>=18.0.0'}
+
   help-me@5.0.0:
     resolution: {integrity: sha512-7xgomUX6ADmcYzFik0HzAxh/73YlKR9bmFzf51CZwR+b6YtzU2m0u49hQCqV6SvlqIqsaxovfwdvbnsw3b/zpg==}
 
@@ -17339,6 +17346,8 @@ snapshots:
 
   he@1.2.0: {}
 
+  helmet@8.1.0: {}
+
   help-me@5.0.0: {}
 
   homedir-polyfill@1.0.3: