fix(BUG-021): remove verification code from API response, send via email

- Replace Resend email service with nodemailer via local postfix relay
- Remove code field from POST /v1/signup/free response
- Send 6-digit verification code via email only (noreply@docfast.dev)
- Add extra_hosts for Docker-to-host SMTP relay
- Fire-and-forget email sending to avoid blocking API response

package.json

@@ -15,12 +15,14 @@
     "helmet": "^8.0.0",
     "marked": "^15.0.0",
     "nanoid": "^5.0.0",
+    "nodemailer": "^8.0.1",
     "puppeteer": "^24.0.0",
     "stripe": "^20.3.1"
   },
   "devDependencies": {
     "@types/express": "^5.0.0",
     "@types/node": "^22.0.0",
+    "@types/nodemailer": "^7.0.9",
     "tsx": "^4.19.0",
     "typescript": "^5.7.0",
     "vitest": "^3.0.0"