Introduce oauth2scope registry with freeze lifecycle

Replace pkg/iam/scopeset with pkg/iam/oauth2scope.Registry, a shared
OAuth2 scope→action registry used by the authorizer, OAuth2 service,
and Connect API. Registration stays open until probod calls Freeze();
read paths (RegisteredScopes, Allows, ValidateScopes) panic before
that.

Drop the leaky APIScopes surface and AllowedAPIScopes on manual
access-token creation in favor of registry.ValidateScopes. Metadata,
protected-resource metadata, and CIMD scope lists are built from
RegisteredScopes() via helpers in pkg/iam/oauth2/scopes.go. Expose
oauth2ScopesSupported as an OAuth2Scope GraphQL scalar.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

pkg/iam/oauth2/protected_resource_metadata_test.go

@@ -21,6 +21,7 @@ import (
 	"github.com/stretchr/testify/require"
 	"go.probo.inc/probo/pkg/coredata"
 	"go.probo.inc/probo/pkg/iam/oauth2"
+	"go.probo.inc/probo/pkg/iam/oauth2scope"
 	"go.probo.inc/probo/pkg/probo"
 	"go.probo.inc/probo/pkg/uri"
 )
@@ -28,12 +29,16 @@ import (
 func TestNewProtectedResourceMetadata(t *testing.T) {
 	t.Parallel()
 
-	apiScopes := []coredata.OAuth2Scope{probo.ScopeV1DocumentRead}
+	reg := oauth2scope.NewRegistry().Register(
+		map[coredata.OAuth2Scope][]string{
+			probo.ScopeV1DocumentRead: {"core:document:get"},
+		},
+	)
 
 	resource := uri.URI("https://app.example.com")
 	authorizationServer := uri.URI("https://app.example.com")
 
-	metadata := oauth2.NewProtectedResourceMetadata(resource, authorizationServer, apiScopes)
+	metadata := oauth2.NewProtectedResourceMetadata(resource, authorizationServer, reg.RegisteredScopes())
 	require.NotNil(t, metadata)
 
 	assert.Equal(t, resource, metadata.Resource)