Add request-scoped context for session.getUser()

Use AsyncLocalStorage to provide request context so services can access
the current user without needing Call passed through every function.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

express/context.ts

@@ -0,0 +1,27 @@
+// context.ts
+//
+// Request-scoped context using AsyncLocalStorage.
+// Allows services to access request data (like the current user) without
+// needing to pass Call through every function.
+
+import { AsyncLocalStorage } from "node:async_hooks";
+import { AnonymousUser, type MaybeUser } from "./user";
+
+type RequestContext = {
+    user: MaybeUser;
+};
+
+const asyncLocalStorage = new AsyncLocalStorage<RequestContext>();
+
+// Run a function within a request context
+function runWithContext<T>(context: RequestContext, fn: () => T): T {
+    return asyncLocalStorage.run(context, fn);
+}
+
+// Get the current user from context, or AnonymousUser if not in a request
+function getCurrentUser(): MaybeUser {
+    const context = asyncLocalStorage.getStore();
+    return context?.user ?? AnonymousUser;
+}
+
+export { getCurrentUser, runWithContext, type RequestContext };