```python
from fastapi import FastAPI
from pydantic import BaseModel
from typing import List, Any
import asyncio
import helpers

app = FastAPI()


class ReportRequest(BaseModel):
    rows: List[Any]


@app.post("/reports")
async def generate_report(request: ReportRequest):
    # Offload the synchronous render_report to a thread to avoid blocking the event loop
    rendered = await asyncio.to_thread(helpers.render_report, request.rows)
    return {"length": len(rendered)}
```