Improve email verification flow and error handling

- Send verification email before creating user to prevent orphaned accounts
- Detect SMTP authentication errors and fail fast without retries
- Add field-level validation errors for better frontend UX
- Configure docker-compose with explicit environment variables
- Document dollar sign escaping in .env.example (use $$)
- Implement welcome email on successful verification
- Clean up unnecessary comments

internal/domain/entities/password_reset_token.go

@@ -0,0 +1,40 @@
+package entities
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type PasswordResetToken struct {
+	ID        string
+	UserID    string
+	Token     string
+	ExpiresAt time.Time
+	UsedAt    *time.Time
+	CreatedAt time.Time
+}
+
+func NewPasswordResetToken(userID, token string, expiresAt time.Time) *PasswordResetToken {
+	return &PasswordResetToken{
+		ID:        uuid.NewString(),
+		UserID:    userID,
+		Token:     token,
+		ExpiresAt: expiresAt,
+		UsedAt:    nil,
+		CreatedAt: time.Now(),
+	}
+}
+
+func (t *PasswordResetToken) IsExpired() bool {
+	return time.Now().After(t.ExpiresAt)
+}
+
+func (t *PasswordResetToken) IsUsed() bool {
+	return t.UsedAt != nil
+}
+
+func (t *PasswordResetToken) MarkAsUsed() {
+	now := time.Now()
+	t.UsedAt = &now
+}