feat(oauth): add OAUTH_AUTHORIZE_URL to override authorize endpoint

Lets a per-brand authorize host front a single canonical issuer, so the
IdP token's `iss` stays constant for downstream validation while login
branding varies per domain. Discovery, token exchange and refresh keep
using OAUTH_ISSUER_URL.

.env.example

@@ -49,6 +49,11 @@ JMAP_SERVER_URL=https://your-jmap-server.com
 # OpenID Connect issuer URL for discovery
 # OAUTH_ISSUER_URL=https://your-idp.example.com
 
+# Overrides only the user-facing authorize endpoint (e.g. a per-brand login
+# host). Discovery, token exchange and refresh keep using OAUTH_ISSUER_URL.
+# Leave unset to use the authorization_endpoint from discovery.
+# OAUTH_AUTHORIZE_URL=https://login.your-brand.example.com/application/o/authorize/
+
 # Allow OAuth discovery to resolve to private (RFC-1918 / loopback) addresses.
 # Off by default as an SSRF guard. Enable for split-DNS deployments where the
 # OAuth issuer's public hostname resolves to an internal IP from this server.

app/api/auth/sso/start/route.ts

@@ -8,6 +8,7 @@ import { discoverOAuth } from '@/lib/oauth/discovery';
 import { getOauthScopes } from '@/lib/oauth/tokens';
 import { getCookieOptions } from '@/lib/oauth/cookie-config';
 import { hasSessionSecret } from '@/lib/auth/session-secret';
+import { configManager } from '@/lib/admin/config-manager';
 
 const SSO_PENDING_COOKIE = 'sso_pending';
 const SSO_PENDING_MAX_AGE = 300; // 5 minutes
@@ -102,8 +103,12 @@ export async function POST(request: NextRequest) {
       maxAge: SSO_PENDING_MAX_AGE,
     });
 
-    // Build authorize URL
-    const authUrl = new URL(metadata.authorization_endpoint);
+    // Build authorize URL. OAUTH_AUTHORIZE_URL, when set, overrides only the
+    // user-facing authorize endpoint (e.g. a per-brand login host). Discovery,
+    // token exchange and refresh keep using the canonical discovered endpoints.
+    const authorizeOverride =
+      configManager.get<string>('oauthAuthorizeUrl', '') || process.env.OAUTH_AUTHORIZE_URL;
+    const authUrl = new URL(authorizeOverride?.trim() || metadata.authorization_endpoint);
     authUrl.searchParams.set('response_type', 'code');
     authUrl.searchParams.set('client_id', clientId);
     authUrl.searchParams.set('redirect_uri', redirect_uri);

lib/admin/types.ts

@@ -163,6 +163,11 @@ export const CONFIG_ENV_MAP: Record<string, { envVar: string; fileEnvVar?: strin
   oauthClientId: { envVar: 'OAUTH_CLIENT_ID', type: 'string', defaultValue: '' },
   oauthClientSecret: { envVar: 'OAUTH_CLIENT_SECRET', fileEnvVar: 'OAUTH_CLIENT_SECRET_FILE', type: 'string', defaultValue: '' },
   oauthIssuerUrl: { envVar: 'OAUTH_ISSUER_URL', type: 'url', defaultValue: '' },
+  // Overrides only the user-facing authorize endpoint. Discovery, token exchange
+  // and refresh continue to use the canonical OAUTH_ISSUER_URL. Lets a per-brand
+  // authorize host front a single canonical issuer. Empty = use the discovered
+  // authorization_endpoint.
+  oauthAuthorizeUrl: { envVar: 'OAUTH_AUTHORIZE_URL', type: 'url', defaultValue: '' },
   oauthScopes: { envVar: 'OAUTH_SCOPES', type: 'string', defaultValue: '' },
   oauthExtraScopes: { envVar: 'OAUTH_EXTRA_SCOPES', type: 'string', defaultValue: '' },
   oauthAllowPrivateEndpoints: { envVar: 'OAUTH_ALLOW_PRIVATE_ENDPOINTS', type: 'boolean', defaultValue: false },