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_test.go

@@ -0,0 +1,48 @@
+package main
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestParseAccounts(t *testing.T) {
+	f, err := os.Open("testdata/accounts.cf")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	accounts, err := parseAccounts(f)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(accounts) != 5 {
+		t.Fatalf("got %d accounts, want 5", len(accounts))
+	}
+	want := []string{
+		"info@produktor.io",
+		"andriy.oblivantsev@produktor.io",
+		"postmaster@produktor.io",
+		"postman@produktor.io",
+		"ano@produktor.io",
+	}
+	for i, w := range want {
+		if accounts[i].Email != w {
+			t.Errorf("account %d = %q, want %q", i, accounts[i].Email, w)
+		}
+	}
+	if accounts[0].Hash == "" {
+		t.Error("expected a hash to be parsed")
+	}
+}
+
+func TestParseAccountsSkipsMalformed(t *testing.T) {
+	in := "# comment\n\nalice@example.org|hash1\nbroken-line-no-pipe\n|hash-no-email\n"
+	accounts, err := parseAccounts(strings.NewReader(in))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(accounts) != 1 || accounts[0].Email != "alice@example.org" {
+		t.Fatalf("got %+v, want only alice@example.org", accounts)
+	}
+}