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>

Dockerfile

@@ -31,10 +31,14 @@ ENV NEXTAUTH_SECRET="build-time-secret"
 ENV IP_SALT="build-time-salt"
 ENV STRIPE_SECRET_KEY="sk_test_placeholder_for_build"
 ENV RESEND_API_KEY="re_placeholder_for_build"
-# Marketing host vs app host. NEXT_PUBLIC_WWW_URL must stay on www: it is the origin
-# encoded into downloaded QR codes and used for public email links.
-ENV NEXT_PUBLIC_WWW_URL="https://www.qrmaster.net"
-ENV NEXT_PUBLIC_APP_URL="https://app.qrmaster.net"
+# Marketing host vs app host. NEXT_PUBLIC_WWW_URL must stay on www in production: it is the
+# origin encoded into downloaded QR codes and used for public email links.
+# Declared as ARG so the staging overlay can build the same image pointing at
+# testmodul.qrmaster.net - the defaults keep a plain production build unchanged.
+ARG NEXT_PUBLIC_WWW_URL="https://www.qrmaster.net"
+ENV NEXT_PUBLIC_WWW_URL=$NEXT_PUBLIC_WWW_URL
+ARG NEXT_PUBLIC_APP_URL="https://app.qrmaster.net"
+ENV NEXT_PUBLIC_APP_URL=$NEXT_PUBLIC_APP_URL
 # PostHog Analytics - REQUIRED at build time for client-side bundle
 ENV NEXT_PUBLIC_POSTHOG_KEY="phc_97JBJVVQlqqiZuTVRHuBnnG9HasOv3GSsdeVjossizJ"
 ENV NEXT_PUBLIC_POSTHOG_HOST="https://us.i.posthog.com"
@@ -50,6 +54,10 @@ ENV NEXT_PUBLIC_UMAMI_ID=$NEXT_PUBLIC_UMAMI_ID
 # middleware and the route handlers disagreeing about the cookie scope.
 ARG COOKIE_DOMAIN=""
 ENV COOKIE_DOMAIN=$COOKIE_DOMAIN
+# Distinct session cookie name for the staging deployment, so its cookie cannot collide
+# with the production one the browser also sends to testmodul.qrmaster.net.
+ARG AUTH_COOKIE_NAME=""
+ENV AUTH_COOKIE_NAME=$AUTH_COOKIE_NAME
 RUN npx prisma generate
 RUN npm run build