fix: Google Sign-In - Firebase Admin credentials via env var + mobile redirect flow

backend/app/api/auth/__init__.py

@@ -1,4 +1,5 @@
 import os
+import json
 import shutil
 from uuid import uuid4
 from typing import Annotated, Optional
@@ -12,11 +13,26 @@ from app.schemas.user import PassengerCreate, Token, UserResponse, LoginRequest,
 import firebase_admin
 from firebase_admin import auth as firebase_auth, credentials
 
-# Initialize Firebase Admin
+# Initialize Firebase Admin SDK
+# Supports two methods:
+#   1. FIREBASE_SERVICE_ACCOUNT_JSON env var with the full JSON as a string (recommended for Render/production)
+#   2. GOOGLE_APPLICATION_CREDENTIALS env var pointing to a JSON file path (local dev)
 try:
     if not firebase_admin._apps:
-        # Default initialization (uses GOOGLE_APPLICATION_CREDENTIALS)
-        firebase_admin.initialize_app()
+        sa_json = os.environ.get("FIREBASE_SERVICE_ACCOUNT_JSON")
+        if sa_json:
+            # Parse the JSON string directly from environment variable
+            sa_dict = json.loads(sa_json)
+            cred = credentials.Certificate(sa_dict)
+            firebase_admin.initialize_app(cred)
+            print("DEBUG: Firebase Admin initialized from FIREBASE_SERVICE_ACCOUNT_JSON env var")
+        elif os.environ.get("GOOGLE_APPLICATION_CREDENTIALS"):
+            # Use file path from GOOGLE_APPLICATION_CREDENTIALS
+            firebase_admin.initialize_app()
+            print("DEBUG: Firebase Admin initialized from GOOGLE_APPLICATION_CREDENTIALS file")
+        else:
+            print("WARNING: No Firebase credentials found. Set FIREBASE_SERVICE_ACCOUNT_JSON or GOOGLE_APPLICATION_CREDENTIALS.")
+            firebase_admin.initialize_app()
 except Exception as e:
     print(f"WARNING: Firebase Admin could not be initialized: {e}")