fix: harden workspace runtime snapshots

backend/src/app.ts

@@ -1,5 +1,6 @@
 import Fastify, { type FastifyInstance } from "fastify";
 import cors from "@fastify/cors";
+import { join } from "node:path";
 import type { AppConfig } from "./config.js";
 import { ThtRunner } from "./tht/tht-runner.js";
 import { PiProcessManager } from "./pi/pi-process-manager.js";
@@ -40,6 +41,7 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
     harnessDir: config.harnessDir,
     configPath: process.env.THT_CONFIG ?? "config/tht.yaml",
     dataRoot: config.dataRoot,
+    runtimeSnapshotRoot: join(config.workspaceRegistry.root, "snapshots", "runtime"),
     secretsFile: config.secretsFile,
     secretFiles: config.secretFiles,
   });