Make the session cookie name configurable for a staging deployment

Groundwork for testmodul.qrmaster.net, a second stack running the `test` branch on a real
qrmaster.net subdomain.

Production scopes its session cookie to .qrmaster.net, so the browser sends it to every
subdomain including staging. With both environments naming the cookie `userId`, the
browser holds two cookies of the same name and cookies.get() picks one arbitrarily -
staging logins would look randomly signed-out. AUTH_COOKIE_NAME lets staging pick
`userId_test` instead. Production keeps the `userId` default; changing it there would
invalidate every existing session.

Wired getAuthCookieName() into the six places that named the cookie literally. The account
deletion route now expires both the host-only and the domain-scoped variant like the logout
route already does, instead of a single cookies().delete() that would leave the other one
behind.

NEXT_PUBLIC_WWW_URL and NEXT_PUBLIC_APP_URL become build ARGs so the same image can be
built pointing at the staging host - the defaults keep a plain production build byte
identical to before. Like COOKIE_DOMAIN these must exist at build time, because process.env
is inlined into the Edge middleware bundle.

robots.ts now serves Disallow-all unless NEXT_PUBLIC_INDEXABLE is true. Staging otherwise
returns the production robots.txt and invites crawlers to index a duplicate of www.

docker-compose.test.yml is the staging overlay. Two things it must get right, both verified
against `docker compose config`:

- db and redis need `networks: !override`. Compose MERGES the networks mapping from the base
  file, and since qrmaster-network is external and shared, a plain list left them attached
  to it - `db` would then resolve to two containers and staging could read and write the
  production database.
- The web entrypoint is replaced so `prisma migrate deploy` never runs. prisma/migrations
  stopped in April 2026 and the schema has moved on through manual SQL since, so applying
  them to a fresh database would build a stale schema. Staging gets its schema from
  `pg_dump --schema-only` against production instead.

Verified: tsc clean, production build succeeds, and the merged compose config confirms
staging keeps db/redis off the shared network while production resolves unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/lib/session.ts

@@ -1,7 +1,7 @@
 import 'server-only';
 import crypto from 'crypto';
 import { cookies } from 'next/headers';
-import { getAuthCookieOptions } from './cookieConfig';
+import { getAuthCookieName, getAuthCookieOptions } from './cookieConfig';
 
 /**
  * Signed session cookie.
@@ -11,8 +11,6 @@ import { getAuthCookieOptions } from './cookieConfig';
  * detect a tampered/forged cookie and reject it. Format: `<userId>.<signature>`.
  */
 
-export const AUTH_COOKIE_NAME = 'userId';
-
 function getSecret(): string {
   const secret = process.env.NEXTAUTH_SECRET;
   if (!secret) {
@@ -68,12 +66,12 @@ export function verifySignedUserId(value: string | undefined | null): string | n
  * Use this in route handlers instead of reading the `userId` cookie directly.
  */
 export function getSessionUserId(): string | null {
-  return verifySignedUserId(cookies().get(AUTH_COOKIE_NAME)?.value);
+  return verifySignedUserId(cookies().get(getAuthCookieName())?.value);
 }
 
 /**
  * Set the signed auth cookie for the given user id (server component / route handler context).
  */
 export function setSessionCookie(userId: string): void {
-  cookies().set(AUTH_COOKIE_NAME, signUserId(userId), getAuthCookieOptions());
+  cookies().set(getAuthCookieName(), signUserId(userId), getAuthCookieOptions());
 }