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/routes/oidc/[...].ts

@@ -0,0 +1,30 @@
+/**
+ * Catch-all route that delegates all /oidc/* requests to the oidc-provider.
+ *
+ * This exposes the standard OIDC endpoints:
+ *   /oidc/auth          — authorization
+ *   /oidc/token         — token exchange
+ *   /oidc/me            — userinfo
+ *   /oidc/session/end   — logout
+ *   /oidc/jwks          — JSON Web Key Set
+ */
+import { getOidcProvider } from "../../utils/oidc-provider.js";
+
+export default defineEventHandler(async (event) => {
+  const provider = await getOidcProvider();
+  const { req, res } = event.node;
+
+  // oidc-provider expects paths relative to its own mount point.
+  // Nitro gives us the full path, so strip the /oidc prefix.
+  const originalUrl = req.url || "";
+  req.url = originalUrl.replace(/^\/oidc/, "") || "/";
+
+  // Hand off to oidc-provider's Connect-style callback
+  const callback = provider.callback() as Function;
+  await new Promise<void>((resolve, reject) => {
+    callback(req, res, (err: unknown) => {
+      if (err) reject(err);
+      else resolve();
+    });
+  });
+});