feat(backend): Ollama embeddings preflight on session create/resume (503 hard-fail)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/app.ts

@@ -43,7 +43,10 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
 
   app.addHook("preHandler", authPreHandler(config.authMode));
   app.get("/health", async () => ({ status: "ok" }));
-  sessionRoutes(app, { mgr, tht: tht as ThtRunner, hub, getSettings });
+  sessionRoutes(app, {
+    mgr, tht: tht as ThtRunner, hub, getSettings,
+    ollamaEnsureTimeoutSec: Math.round(config.ollamaEnsureTimeoutMs / 1000),
+  });
   sqlRoutes(app, { tht: tht as ThtRunner });
   metaRoutes(app, { harnessDir: config.harnessDir, listModels });
   settingsRoutes(app, { cfg: config, listModels });