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/repositories/user_repository.go

@@ -12,4 +12,5 @@ type UserRepository interface {
 	FindByEmail(ctx context.Context, email string) (*entities.User, error)
 	FindByVerificationToken(ctx context.Context, token string) (*entities.User, error)
 	Update(ctx context.Context, user *entities.User) error
+	Delete(ctx context.Context, id string) error
 }