```python
import asyncio
from fastapi import FastAPI, Request

import helpers  # noqa: F401

app = FastAPI()

@app.post("/reports")
async def generate_report(request: Request):
    data = await request.json()
    rows = data.get("rows")
    loop = asyncio.get_event_loop()
    rendered = await loop.run_in_executor(None, helpers.render_report, rows)
    return {"length": len(rendered)}
```