feat: add email service with Nodemailer and 6 HTML templates

EmailService with mock fallback when SMTP not configured.
Templates: welcome, fiel-notification, payment-confirmed,
payment-failed, subscription-expiring, subscription-cancelled.
Uses Google Workspace SMTP (STARTTLS).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/package.json

@@ -30,6 +30,7 @@
     "jsonwebtoken": "^9.0.2",
     "node-cron": "^4.2.1",
     "node-forge": "^1.3.3",
+    "nodemailer": "^8.0.2",
     "pg": "^8.18.0",
     "zod": "^3.23.0"
   },
@@ -42,6 +43,7 @@
     "@types/node": "^22.0.0",
     "@types/node-cron": "^3.0.11",
     "@types/node-forge": "^1.3.14",
+    "@types/nodemailer": "^7.0.11",
     "@types/pg": "^8.18.0",
     "prisma": "^5.22.0",
     "tsx": "^4.19.0",