Ensure `/users/me` endpoint returns complete user info

[3nd3r1]

Verify and enhance the existing /api/v1/users/me endpoint to return all necessary user information for display.

Technical Details:

• Review app/api/v1/endpoints/users.py
• Ensure endpoint returns: id, email, username, created_at
• Update UserResponse schema if needed
• Add integration test for authenticated user info retrieval
• Document endpoint in API docs

Acceptance Criteria:

• Endpoint returns user id, email, and username
• Requires valid JWT token
• Returns 401 if token is missing or invalid
• Integration tests pass
• API documentation is up to date