app/services/firebase.py

import firebase_admin
from firebase_admin import credentials, firestore
import json
from app.config import FIREBASE_SERVICE_ACCOUNT

if not firebase_admin._apps:
    if not FIREBASE_SERVICE_ACCOUNT:
        raise RuntimeError("Missing FIREBASE_SERVICE_ACCOUNT")

    cred = credentials.Certificate(json.loads(FIREBASE_SERVICE_ACCOUNT))
    firebase_admin.initialize_app(cred)

db = firestore.client()