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/accounts.go

@@ -0,0 +1,37 @@
+// Package main implements mail-admin, a read-only web view of the
+// docker-mailserver accounts on mail.produktor.io.
+package main
+
+import (
+	"bufio"
+	"io"
+	"strings"
+)
+
+// Account is one mailbox from postfix-accounts.cf.
+type Account struct {
+	Email string // full address, e.g. info@produktor.io
+	Hash  string // SHA512-CRYPT hash from the accounts file (never displayed)
+}
+
+// parseAccounts reads a docker-mailserver postfix-accounts.cf file: one
+// "email|hash" per line, '#' comments and blank lines skipped. Malformed
+// lines are skipped, not fatal: a broken line must not hide the other
+// accounts.
+func parseAccounts(r io.Reader) ([]Account, error) {
+	sc := bufio.NewScanner(r)
+	var out []Account
+	for sc.Scan() {
+		line := strings.TrimSpace(sc.Text())
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		email, hash, ok := strings.Cut(line, "|")
+		email = strings.TrimSpace(email)
+		if !ok || email == "" {
+			continue
+		}
+		out = append(out, Account{Email: email, Hash: strings.TrimSpace(hash)})
+	}
+	return out, sc.Err()
+}