Add OIDC provider for Outline wiki SSO

Add oidc-provider with MongoDB adapter so ghostguild.org can act as
the identity provider for the self-hosted Outline wiki. Members
authenticate via the existing magic-link flow, with automatic SSO
when an active session exists. Includes interaction routes, well-known
discovery endpoint, and login page.

server/middleware/01.csrf.js

@@ -7,6 +7,7 @@ const EXEMPT_PREFIXES = [
   '/api/helcim/webhook',
   '/api/slack/webhook',
   '/api/auth/verify',
+  '/oidc/',
 ]
 
 function isExempt(path) {