feat: harden runtime readiness and session workflow

backend/test/health.test.ts

@@ -38,6 +38,7 @@ test("SSE response headers are flushed before the first event", async () => {
     ]);
     expect(response.headers.get("content-type")).toContain("text/event-stream");
     expect(response.headers.get("cache-control")).toBe("no-cache");
+    expect(response.headers.get("x-accel-buffering")).toBe("no");
   } finally {
     controller.abort();
     await app.close();