feat: SMTP, maintenance mode, blog topic bank UI

8. SMTP: emailService.js (nodemailer), templates (welcome/payment/low_credits)
   /api/admin/email/test — тест отправки
   app_settings category=smtp (HOST/PORT/USER/PASS/FROM/ENABLED)
9. Maintenance mode: middleware в index.js, MAINTENANCE_MODE в engine settings
   При true → 503 для всех запросов кроме /uploads и /api/settings
10. Blog topic bank:
   DB: blog_topics(category,topic,is_used,source,priority)
   40 тем мигрированы из хардкода (source=hardcoded)
   autogen.js: getNextTopic берёт из DB, fallback на TOPIC_BANK
   admin API: GET/POST /blog-topics, DELETE /:id, POST /generate (AI +10)

package.json

@@ -23,6 +23,7 @@
     "fast-xml-parser": "^4.5.6",
     "ioredis": "^5.11.0",
     "node-cron": "^4.2.1",
+    "nodemailer": "^8.0.11",
     "pg": "^8.21.0",
     "sharp": "^0.34.5"
   }