Replace `requests` with `httpx` in SMS service

Problem

app/services/sms_service.py uses the synchronous requests library to call the Fast2SMS API. In an async FastAPI application this blocks the event loop for the duration of the HTTP call, degrading throughput under load.

Location

app/services/sms_service.py

Fix

Replace with httpx (BSD-3, async-native, already the standard in FastAPI projects):

import httpx

async def send_otp_sms(phone: str, otp: str):
    async with httpx.AsyncClient() as client:
        response = await client.post(FAST2SMS_URL, ...)

httpx supports retries via httpx-retries and has a nearly identical API to requests.

References

• https://www.python-httpx.org/

Effort

Low (~1h)