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/server/api/connect/v1/resolver.go

@@ -40,6 +40,7 @@ import (
 	"go.probo.inc/probo/pkg/filemanager"
 	"go.probo.inc/probo/pkg/gid"
 	"go.probo.inc/probo/pkg/iam"
+	"go.probo.inc/probo/pkg/iam/oauth2scope"
 	"go.probo.inc/probo/pkg/saferedirect"
 	"go.probo.inc/probo/pkg/securecookie"
 	"go.probo.inc/probo/pkg/server/api/authn"
@@ -53,6 +54,7 @@ type (
 		batchAuthorize authz.BatchAuthorizeFunc
 		logger         *log.Logger
 		iam            *iam.Service
+		scopeRegistry  *oauth2scope.Registry
 		fileManager    *filemanager.Service
 		baseURL        *baseurl.BaseURL
 		sessionCookie  *authn.Cookie