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-lock.json

@@ -13,12 +13,14 @@
         "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"
@@ -984,6 +986,15 @@
         "undici-types": "~6.21.0"
       }
     },
+    "node_modules/@types/nodemailer": {
+      "version": "7.0.9",
+      "resolved": "https://registry.npmjs.org/@types/nodemailer/-/nodemailer-7.0.9.tgz",
+      "integrity": "sha512-vI8oF1M+8JvQhsId0Pc38BdUP2evenIIys7c7p+9OZXSPOH5c1dyINP1jT8xQ2xPuBUXmIC87s+91IZMDjH8Ow==",
+      "dev": true,
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/@types/qs": {
       "version": "6.14.0",
       "resolved": "https://registry.npmjs.org/@types/qs/-/qs-6.14.0.tgz",
@@ -2572,6 +2583,14 @@
         "node": ">= 0.4.0"
       }
     },
+    "node_modules/nodemailer": {
+      "version": "8.0.1",
+      "resolved": "https://registry.npmjs.org/nodemailer/-/nodemailer-8.0.1.tgz",
+      "integrity": "sha512-5kcldIXmaEjZcHR6F28IKGSgpmZHaF1IXLWFTG+Xh3S+Cce4MiakLtWY+PlBU69fLbRa8HlaGIrC/QolUpHkhg==",
+      "engines": {
+        "node": ">=6.0.0"
+      }
+    },
     "node_modules/object-inspect": {
       "version": "1.13.4",
       "resolved": "https://registry.npmjs.org/object-inspect/-/object-inspect-1.13.4.tgz",