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/probo/service.go

@@ -149,7 +149,6 @@ func NewService(
 	}
 
 	iamService.Authorizer.RegisterPolicySet(ProboPolicySet())
-	iamService.OAuth2ScopeSet.Register(OAuth2ScopeMappings)
 
 	svc := &Service{
 		pg:                      pgClient,