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-lock.json

@@ -18,6 +18,7 @@
         "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"
       }
@@ -1661,6 +1662,15 @@
         "node-gyp-build-optional-packages-test": "build-test.js"
       }
     },
+    "node_modules/nodemailer": {
+      "version": "8.0.11",
+      "resolved": "https://registry.npmjs.org/nodemailer/-/nodemailer-8.0.11.tgz",
+      "integrity": "sha512-nrO/pDAUKl+wXX+lx16tDLbnm0fW6sK/x8mgohaCpg+CdCEl482bD4tCuAZk2DyliruiNTIZxRCoWkDqJEnAiA==",
+      "license": "MIT-0",
+      "engines": {
+        "node": ">=6.0.0"
+      }
+    },
     "node_modules/nth-check": {
       "version": "2.1.1",
       "resolved": "https://registry.npmjs.org/nth-check/-/nth-check-2.1.1.tgz",