refactor: lru_cache fuer get_settings, vollere field-coverage in tests

app/config.py

@@ -1,3 +1,5 @@
+from functools import lru_cache
+
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
@@ -22,11 +24,6 @@ class Settings(BaseSettings):
     log_level: str = "INFO"
 
 
-_settings: Settings | None = None
-
-
+@lru_cache(maxsize=1)
 def get_settings() -> Settings:
-    global _settings
-    if _settings is None:
-        _settings = Settings()
-    return _settings
+    return Settings()