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/main.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"log"
+	"net/http"
+	"os"
+)
+
+// env returns the value of key or def when unset/empty.
+func env(key, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return def
+}
+
+// envRequired returns the value of key or exits: a missing credential must
+// fail loudly, never run with an open door.
+func envRequired(key string) string {
+	v := os.Getenv(key)
+	if v == "" {
+		log.Fatalf("mail-admin: %s is required (set it in mail-server/.env)", key)
+	}
+	return v
+}
+
+func main() {
+	s := &server{
+		accountsPath: envRequired("MAIL_ADMIN_ACCOUNTS"),
+		quotaPath:    envRequired("MAIL_ADMIN_QUOTAS"),
+		maildirRoot:  envRequired("MAIL_ADMIN_MAILDIR"),
+		basePath:     env("MAIL_ADMIN_BASE_PATH", ""),
+		user:         envRequired("MAIL_ADMIN_USER"),
+		pass:         envRequired("MAIL_ADMIN_PASSWORD"),
+	}
+	addr := env("MAIL_ADMIN_LISTEN", ":8080")
+	log.Printf("mail-admin listening on %s, base path %q", addr, s.basePath)
+	log.Fatal(http.ListenAndServe(addr, s.handler()))
+}