Make the admin a member with a flag, and drop the second listener

The admin was a set of env credentials on its own loopback listener. That
bought network isolation, and charged a second port to tunnel and proxy and
a second credential in the password manager. It also sat outside the
SameSite protection the member cookie already had, and left every ban and
password reset with no actor to log.

is_admin on users reuses what was already there: the session, the login
rate limiter, ban-drops-sessions, CSRF. /admin is now a route on the member
mux. A member without the flag gets 404 rather than 403 — the pages are
none of their business, and "forbidden" confirms there is something to be
forbidden from.

Registration needs an invite and invites come from /admin, so an empty
database cannot grow its first user. seedAdmin breaks that circle exactly
once, from ADMIN_EMAIL and ADMIN_PASSWORD, and does nothing against a
database that already has users.

An admin cannot ban themselves: banning drops the target's sessions, and
nothing would be left that could undo it.

This reverses decision 8, which is rewritten rather than deleted, along
with the admin entry in the CONTEXT.md vocabulary.

docker-compose.yml

@@ -5,12 +5,11 @@ services:
       args:
         VERSION: ${VERSION:-dev}
     environment:
-      ADMIN_USER: ${ADMIN_USER:-admin}
-      ADMIN_PASSWORD: ${ADMIN_PASSWORD:?set ADMIN_PASSWORD in .env}
+      # Only used to create the first account on an empty database; inert after that.
+      ADMIN_EMAIL: ${ADMIN_EMAIL:-}
+      ADMIN_PASSWORD: ${ADMIN_PASSWORD:-}
+      ADMIN_NAME: ${ADMIN_NAME:-Ylläpito}
       ADDR: ":8080"
-      # Inside the container the admin listener must bind the container's own interface; it is not
-      # published below, so it stays unreachable from outside without a tunnel or the proxy.
-      ADMIN_ADDR: ":8081"
       SECURE_COOKIES: ${SECURE_COOKIES:-true}
       PUBLIC_URL: ${PUBLIC_URL:-}
     # The SQLite file sits in here beside the audio, so this one mount is the whole backup.
@@ -18,5 +17,4 @@ services:
       - ./storage:/storage
     ports:
       - "8080:8080"
-      - "8081:8081"
     restart: unless-stopped