feat(mail-admin): read-only account view for mail.produktor.io (#74)

- admin/: Go stdlib-only HTTP viewer, lists accounts from
  config/postfix-accounts.cf with per-mailbox message counts (INBOX and
  total, same numbers doveadm reports) and storage; quota from
  dovecot-quotas.cf; Basic Auth from .env; offline tests vs fixtures
  (go test -race ./...)
- compose: mail-admin service, build admin/Dockerfile, publishes
  127.0.0.1:19945 / 172.17.0.1:19945; config and mail-data mounted :ro,
  no docker socket
- NPM proxy host 66: location /admin/ -> 172.17.0.1:19945
- README: account admin section

admin/Dockerfile

@@ -0,0 +1,15 @@
+# mail-admin — read-only web view of the docker-mailserver accounts.
+# Build with the Go toolchain, run as a static binary on scratch.
+FROM golang:1.25-alpine AS build
+WORKDIR /src
+COPY go.mod ./
+COPY *.go ./
+RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /mail-admin .
+
+FROM scratch
+COPY --from=build /mail-admin /mail-admin
+# mail-data files are owned by uid/gid 5000 (docker-mailserver), and the
+# service mounts ./config and ./data/mail-data read-only.
+USER 5000:5000
+EXPOSE 8080
+ENTRYPOINT ["/mail-admin"]