backend: add push token API and FCM delivery pipeline

app/config/settings.py

@@ -39,6 +39,10 @@ class Settings(BaseSettings):
     smtp_use_ssl: bool = False
     smtp_timeout_seconds: float = 10.0
     smtp_from_email: str = "no-reply@benyamessenger.local"
+    firebase_enabled: bool = False
+    firebase_credentials_path: str | None = None
+    firebase_credentials_json: str | None = None
+    firebase_webpush_link: str = "https://chat.daemonlord.ru/"
 
     login_rate_limit_per_minute: int = 10
     register_rate_limit_per_minute: int = 5