```python
from fastapi import FastAPI, Query
import asyncio
import helpers  # noqa: F401

app = FastAPI()

@app.get("/portfolio")
async def portfolio(symbols: str = Query(...)):
    # Parse comma-separated symbols into a list, stripping any whitespace
    symbol_list = [s.strip() for s in symbols.split(",")]
    # Launch all async fetches concurrently
    tasks = [helpers.afetch_price(sym) for sym in symbol_list]
    results = await asyncio.gather(*tasks)
    # Build the response dictionary
    prices = {sym: price for sym, price in zip(symbol_list, results)}
    return {"prices": prices}
```