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

docker-compose.yml

@@ -5,6 +5,8 @@ services:
     restart: unless-stopped
     ports:
       - "127.0.0.1:3100:3100"
+    extra_hosts:
+      - "host.docker.internal:host-gateway"
     environment:
       - API_KEYS=${API_KEYS}
       - PORT=3100
@@ -13,7 +15,8 @@ services:
       - STRIPE_WEBHOOK_SECRET=${STRIPE_WEBHOOK_SECRET}
       - BASE_URL=${BASE_URL:-https://docfast.dev}
       - PRO_KEYS=${PRO_KEYS}
-      - RESEND_API_KEY=${RESEND_API_KEY:-FILL_IN}
+      - SMTP_HOST=host.docker.internal
+      - SMTP_PORT=25
     volumes:
       - docfast-data:/app/data
     mem_limit: 512m