#17 Fix, OAuth registration support to any MCP endpoint

[darthvader58]

Problem

Chatgpt developer mode was failing to register custom OAuth connectors with the MCP endpoint https://api.supermemory.ai/mcp, returning a validation error:

Validation error for RegisterOAuthClientResponse 
Input should be a valid dictionary or instance of RegisterOAuthClientResponse 
[type=model_type, input_value='{"client_id":"gQIWiUdSvz...client_name":"ChatGPT"}', input_type=str]

This was simply a ChatGPT-specific compatibility issue where the response was being parsed as a string instead of a JSON object.

Solution

Fixes #17

This PR implements a complete OAuth client registration flow with proper CORS handling:

• Added `/oauth/register` endpoint - Handles ChatGPT's OAuth client registration requests
• Global CORS headers - Ensures all responses work with ChatGPT's browser environment
• Preflight OPTIONS and CORS support- Proper preflight OPTIONS handling and cross-origin headers
• `/mcp` endpoint - The original failing endpoint now supports OAuth registration

Proposed solutions works with any MCP client supporting OAuth 2.0 and supports proper JSON serialization with Content-Type: application/json headers