flatten app to repo root and update docker deployment for single-stack runtime

lib/server/http.ts

@@ -0,0 +1,11 @@
+export function jsonError(message: string, status = 400) {
+  return Response.json({ error: message }, { status });
+}
+
+export function asErrorMessage(error: unknown, fallback: string) {
+  if (error instanceof Error && error.message.trim().length > 0) {
+    return error.message;
+  }
+
+  return fallback;
+}