Implement email/username uniqueness validation in user creation

[coderabbitai]

Overview

Currently, there's no validation to ensure email and username uniqueness when creating new users, which could lead to data integrity issues.

Context

This issue was identified during code review of PR #38:

• PR: Create a migration command to migrate the data from V2 to V3 #38
• Comment: Create a migration command to migrate the data from V2 to V3 #38 (comment)

Requirements

• Add validation before user creation to check if email or username already exists
• Throw appropriate error with context if validation fails
• Update the UserService.create method to include this validation