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.go

@@ -15,8 +15,6 @@
 package oauth2
 
 import (
-	"slices"
-
 	"go.probo.inc/probo/pkg/coredata"
 	"go.probo.inc/probo/pkg/uri"
 )
@@ -33,7 +31,7 @@ type ProtectedResourceMetadata struct {
 func NewProtectedResourceMetadata(
 	resource uri.URI,
 	authorizationServer uri.URI,
-	apiScopes []coredata.OAuth2Scope,
+	registeredScopes []coredata.OAuth2Scope,
 ) *ProtectedResourceMetadata {
 	return &ProtectedResourceMetadata{
 		Resource:             resource,
@@ -41,9 +39,6 @@ func NewProtectedResourceMetadata(
 		BearerMethodsSupported: []string{
 			"header",
 		},
-		ScopesSupported: slices.Concat(
-			[]coredata.OAuth2Scope{ScopeOpenID},
-			apiScopes,
-		),
+		ScopesSupported: protectedResourceScopes(registeredScopes),
 	}
 }