refactor(server): replace AppContext undefined sentinels with Feature<T>

Co-authored-by: Amp <amp@ampcode.com>
Amp-Thread-ID: https://ampcode.com/threads/T-019e5550-4435-7118-8393-cdcc97042178

app/server/app.ts

@@ -36,10 +36,19 @@ try {
 }
 
 const ctx = await createAppContext(config);
-ctx.auth.start();
+ctx.startServices();
 
 export { config };
 
+/**
+ * Disposes the per-process context. Invoked by the production
+ * supervisor on SIGTERM/SIGINT and by the dev Vite plugin on HMR
+ * reload.
+ */
+export async function dispose(): Promise<void> {
+  await ctx.dispose();
+}
+
 // TODO: `getLoadContext` is the right place to handle reverse proxy
 // translation — better than doing it in the OIDC client because it
 // applies to all requests, not just OIDC ones.