backend: add push token API and FCM delivery pipeline

.env.example

@@ -34,6 +34,10 @@ SMTP_USE_TLS=false
 SMTP_USE_SSL=false
 SMTP_TIMEOUT_SECONDS=10
 SMTP_FROM_EMAIL=no-reply@benyamessenger.local
+FIREBASE_ENABLED=false
+FIREBASE_CREDENTIALS_PATH=
+FIREBASE_CREDENTIALS_JSON=
+FIREBASE_WEBPUSH_LINK=https://chat.daemonlord.ru/
 
 LOGIN_RATE_LIMIT_PER_MINUTE=10
 REGISTER_RATE_LIMIT_PER_MINUTE=5