feat:implemented register api endpoint

[aaravpanchalijs-code]

Summary

Implemented the skeleton for the Register API endpoint in the coordination server.

What this PR does

• Adds RegisterHandler
• Parses and validates incoming JSON requests
• Verifies OAuth token using a dummy implementation
• Checks for existing users using dummy data
• Checks for existing devices using dummy data
• Allocates a dummy virtual IP
• Generates a dummy allowlist
• Returns appropriate HTTP status codes and JSON responses

Related issue

Closes #6

Component(s) touched

• Coordination server
• Clientside Agent (WireGuard / NAT / CLI)
• Desktop client (Frontend)
• Docs / Infra

Testing

Tested using Postman.

Verified the following scenarios:

• Successful registration (201 Created)
• Invalid JSON body (400 Bad Request)
• Missing required fields (400 Bad Request)
• Invalid OAuth token (401 Unauthorized)
• User awaiting admin approval (403 Forbidden)
• Device already registered (409 Conflict)

Checklist

• Code follows style standards
• Tests performed manually
• No unrelated files changed

Notes

This implementation currently uses dummy data. Database integration, OAuth verification, and allowlist generation will be connected to the actual backend in future iterations.

API Testing Screenshots

201 Created

400 Bad request

409 Conflict

401 Unauthorized