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/quota_test.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestParseQuotas(t *testing.T) {
+	f, err := os.Open("testdata/quotas.cf")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	quotas, err := parseQuotas(f)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(quotas) != 2 {
+		t.Fatalf("got %d quotas, want 2", len(quotas))
+	}
+	if quotas["info@produktor.io"] != "500M" {
+		t.Errorf("info quota = %q, want 500M", quotas["info@produktor.io"])
+	}
+	if quotas["ano@produktor.io"] != "250M" {
+		t.Errorf("ano quota = %q, want 250M", quotas["ano@produktor.io"])
+	}
+}
+
+func TestParseQuotasEmpty(t *testing.T) {
+	quotas, err := parseQuotas(strings.NewReader("# no limits\n\n"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(quotas) != 0 {
+		t.Fatalf("got %d quotas, want 0", len(quotas))
+	}
+}