Fixes#710
This PR adds CORS middleware to enable browser-based clients and web
applications to access the registry API.
## Changes
- Add CORSMiddleware in internal/api/cors.go to handle preflight OPTIONS
requests and inject CORS headers
- Integrate CORS middleware into the server middleware stack
- Add CORS configuration options (CORS_ENABLED and CORS_ALLOWED_ORIGIN)
## Testing
- All existing tests pass
- Manual testing confirms CORS headers are present in responses
- OPTIONS preflight requests now return 204 with proper headers
## Configuration
New environment variables:
- MCP_REGISTRY_CORS_ENABLED (default: true)
- MCP_REGISTRY_CORS_ALLOWED_ORIGIN (default: *)
