feat: add CORS support to registry API (#711)

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: *)

go.sum

@@ -62,6 +62,8 @@ github.com/prometheus/procfs v0.17.0 h1:FuLQ+05u4ZI+SS/w9+BWEM2TXiHKsUQ9TADiRH7D
 github.com/prometheus/procfs v0.17.0/go.mod h1:oPQLaDAMRbA+u8H5Pbfq+dl3VDAvHxMUOVhe0wYB2zw=
 github.com/rogpeppe/go-internal v1.13.1 h1:KvO1DLK/DRN07sQ1LQKScxyZJuNnedQ5/wKSR38lUII=
 github.com/rogpeppe/go-internal v1.13.1/go.mod h1:uMEvuHeurkdAXX61udpOXGD/AzZDWNMNyH2VO9fmH0o=
+github.com/rs/cors v1.11.1 h1:eU3gRzXLRK57F5rKMGMZURNdIG4EoAmX8k94r9wXWHA=
+github.com/rs/cors v1.11.1/go.mod h1:XyqrcTp5zjWr1wsJ8PIRZssZ8b/WMcMf71DJnit4EMU=
 github.com/santhosh-tekuri/jsonschema/v5 v5.3.1 h1:lZUw3E0/J3roVtGQ+SCrUrg3ON6NgVqpn3+iol9aGu4=
 github.com/santhosh-tekuri/jsonschema/v5 v5.3.1/go.mod h1:uToXkOrWAZ6/Oc07xWQrPOhJotwFIyu2bBVN41fcDUY=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=