fix: avoid blocking the event loop during OAuth token exchange

[kratos0718]

Overview

The Google OAuth callback (src/khoj/routers/auth.py, the async auth route) exchanges the authorization code for a token using a synchronous requests.post:

async def auth(request: Request):
    ...
    verified_data = requests.post("https://oauth2.googleapis.com/token", ...)

A blocking call inside an async handler blocks the entire event loop for the duration of the network round-trip. So during every login's token exchange, all other in-flight requests the server is handling are frozen.

Fix

Offload the blocking call to a worker thread with asyncio.to_thread, keeping the loop responsive:

verified_data = await asyncio.to_thread(
    requests.post,
    "https://oauth2.googleapis.com/token",
    headers={"Content-Type": "application/x-www-form-urlencoded"},
    data=payload,
)

asyncio is already imported in this module; no new dependency, behavior unchanged.

• python -m py_compile clean.