Add OAuth2 API scope registration and enforcement

Register v1 API scopes in coredata, advertise them in OIDC discovery
and protected-resource metadata, show them on the consent screen, and
enforce scope-to-action mapping in the IAM Authorizer before policy
evaluation.

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

pkg/iam/authorizer_oauth2_scope_test.go

@@ -0,0 +1,85 @@
+// 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 iam
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.probo.inc/probo/pkg/coredata"
+	"go.probo.inc/probo/pkg/gid"
+	"go.probo.inc/probo/pkg/iam/oauth2"
+)
+
+func TestAuthorizer_checkOAuth2Scope(t *testing.T) {
+	t.Parallel()
+
+	const scopeV1OrgRead = coredata.OAuth2Scope("v1:org:read")
+
+	principal := gid.New(gid.NilTenant, coredata.IdentityEntityType)
+	action := Action("core:organization:get")
+
+	t.Run("skips when access token is absent", func(t *testing.T) {
+		t.Parallel()
+
+		a := &Authorizer{}
+
+		err := a.checkOAuth2Scope(context.Background(), principal, action)
+		require.NoError(t, err)
+	})
+
+	t.Run("denies before IAM when scopes are not registered", func(t *testing.T) {
+		t.Parallel()
+
+		a := &Authorizer{}
+
+		ctx := oauth2.ContextWithAccessToken(
+			context.Background(),
+			&coredata.OAuth2AccessToken{Scopes: coredata.OAuth2Scopes{scopeV1OrgRead}},
+		)
+
+		err := a.checkOAuth2Scope(ctx, principal, action)
+		require.Error(t, err)
+
+		scopeErr, ok := errors.AsType[*ErrInsufficientOAuth2Scope](err)
+		require.True(t, ok)
+		assert.Equal(t, principal, scopeErr.IdentityID)
+		assert.Equal(t, action, scopeErr.Action)
+	})
+
+	t.Run("allows when registered scopes authorize the action", func(t *testing.T) {
+		t.Parallel()
+
+		a := NewAuthorizer(nil, nil)
+		a.RegisterScopes(
+			CreateScopeSet(
+				map[coredata.OAuth2Scope][]Action{
+					scopeV1OrgRead: {action},
+				},
+			),
+		)
+
+		ctx := oauth2.ContextWithAccessToken(
+			context.Background(),
+			&coredata.OAuth2AccessToken{Scopes: coredata.OAuth2Scopes{scopeV1OrgRead}},
+		)
+
+		err := a.checkOAuth2Scope(ctx, principal, action)
+		require.NoError(t, err)
+	})
+}