feat: add user service layer with business logic

- Create services/user_service.go with user creation and verification
- Add input validation for username, email, and password
- Implement duplicate user checking and password hashing
- Add comprehensive error handling and logging

services/email_service.go

@@ -0,0 +1,34 @@
+package services
+
+import (
+	"fmt"
+	"net/smtp"
+	"os"
+)
+
+type EmailService struct {
+	smtpHost     string
+	smtpPort     string
+	smtpUser     string
+	smtpPassword string
+}
+
+func NewEmailService() *EmailService {
+	return &EmailService{
+		smtpHost:     os.Getenv("SMTP_SERVER"),
+		smtpPort:     os.Getenv("SMTP_PORT"),
+		smtpUser:     os.Getenv("SMTP_USER"),
+		smtpPassword: os.Getenv("SMTP_PASSWORD"),
+	}
+}
+
+func (e *EmailService) SendEmail(to, subject, body string) error {
+	from := e.smtpUser
+
+	msg := fmt.Sprintf("From: %s\r\nTo: %s\r\nSubject: %s\r\n\r\n%s", from, to, subject, body)
+
+	auth := smtp.PlainAuth("", e.smtpUser, e.smtpPassword, e.smtpHost)
+	addr := fmt.Sprintf("%s:%s", e.smtpHost, e.smtpPort)
+
+	return smtp.SendMail(addr, auth, from, []string{to}, []byte(msg))
+}