Add OAuth client branding for trust centers

Serve CIMD logos from dedicated endpoints, expose client metadata
branding through GraphQL, and resolve branding from OAuth clients.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/iam/oauth2/client_branding_test.go

@@ -0,0 +1,66 @@
+// 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 oauth2
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.gearno.de/kit/log"
+	"go.probo.inc/probo/pkg/coredata"
+	"go.probo.inc/probo/pkg/uri"
+)
+
+func TestClientBrandingFromClient(t *testing.T) {
+	t.Parallel()
+
+	clientURL := uri.URI("https://example.com")
+	logoURL := uri.URI("https://example.com/logo.png")
+
+	branding := ClientBrandingFromClient(
+		&coredata.OAuth2Client{
+			ClientName: "Acme",
+			ClientURI:  &clientURL,
+			LogoURI:    &logoURL,
+		},
+	)
+
+	require.NotNil(t, branding)
+	assert.Equal(t, "Acme", branding.Name)
+	assert.Equal(t, "https://example.com", *branding.ClientURL)
+	assert.Equal(t, "https://example.com/logo.png", *branding.LogoURL)
+}
+
+func TestClientBranding_EmptyClientID(t *testing.T) {
+	t.Parallel()
+
+	svc := NewService(nil, nil, "", log.NewLogger())
+
+	branding, err := svc.ClientBranding(context.Background(), "")
+	require.NoError(t, err)
+	assert.Nil(t, branding)
+}
+
+func TestClientBranding_InvalidClientID(t *testing.T) {
+	t.Parallel()
+
+	svc := NewService(nil, nil, "", log.NewLogger())
+
+	branding, err := svc.ClientBranding(context.Background(), "not-a-cimd-client")
+	require.NoError(t, err)
+	assert.Nil(t, branding)
+}