feat(lexware): add integration foundation

backend/hermes/app/core/config.py

@@ -32,6 +32,9 @@ class Settings(BaseSettings):
     smtp_from_email: str | None = None
     smtp_from_name: str = "Funktechnik Schubert"
     smtp_use_tls: bool = True
+    lexware_enabled: bool = False
+    lexware_api_base_url: str = "https://api.lexware.io"
+    lexware_api_key: str | None = None
 
     model_config = SettingsConfigDict(
         env_file=".env",