feat: SMTP mail stack and separate server.env deploy

Add mail.php with STARTTLS sending, test scripts, server.env split from
local .env, admin@ postbox docs, DKIM DNS support-ticket notes, and todo 11
for boka relay.

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/test-mail.php

@@ -0,0 +1,39 @@
+#!/usr/bin/env php
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Send a test mail using local .env (this machine only).
+ * Usage: php scripts/test-mail.php
+ */
+
+$root = dirname(__DIR__);
+require_once "{$root}/config/mail.php";
+
+$to = env('ALERT_EMAIL');
+if (!$to) {
+    fwrite(STDERR, "ALERT_EMAIL not set in .env\n");
+    exit(1);
+}
+
+$from = env('SMTP_FROM', env('SMTP_USER'));
+$domain = env('DOMAIN', '0012.de');
+
+$subject = "[{$domain}] SMTP test (local) " . gmdate('Y-m-d H:i:s') . ' UTC';
+$body = implode("\n", [
+    '0012.de SMTP test — sent from local machine',
+    '',
+    'From header: ' . $from,
+    'SMTP auth: ' . env('SMTP_USER'),
+    '',
+    'If you receive this, local SMTP settings work.',
+]);
+
+try {
+    sendMail($to, $subject, $body);
+    echo "Sent test mail to {$to} (From: {$from})\n";
+} catch (Throwable $e) {
+    fwrite(STDERR, 'Failed: ' . $e->getMessage() . "\n");
+    exit(1);
+}