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/gqlutils/types/oauth2scope/scope.go

@@ -0,0 +1,43 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package oauth2scope
+
+import (
+	"fmt"
+	"io"
+	"strconv"
+
+	"github.com/99designs/gqlgen/graphql"
+	"go.probo.inc/probo/pkg/coredata"
+)
+
+type OAuth2ScopeScalar = coredata.OAuth2Scope
+
+func MarshalOAuth2ScopeScalar(s OAuth2ScopeScalar) graphql.Marshaler {
+	return graphql.WriterFunc(
+		func(w io.Writer) {
+			_, _ = w.Write([]byte(strconv.Quote(s.String())))
+		},
+	)
+}
+
+func UnmarshalOAuth2ScopeScalar(v any) (OAuth2ScopeScalar, error) {
+	s, ok := v.(string)
+	if !ok {
+		return "", fmt.Errorf("oauth2 scope must be a string")
+	}
+
+	return OAuth2ScopeScalar(s), nil
+}