Improve test coverance and fixes according to tests

app/tests/test_config.py

@@ -0,0 +1,16 @@
+from app.core.config import get_settings
+
+def test_get_settings_returns_same_instance():
+    s1 = get_settings()
+    s2 = get_settings()
+
+    assert s1 is s2  # lru_cache should return same instance
+
+def test_default_settings_values():
+    settings = get_settings()
+
+    assert settings.app_name == "URL Shortener"
+    assert settings.host == "0.0.0.0"
+    assert settings.port == 8000
+    assert settings.debug is True
+    assert isinstance(settings.allowed_hosts, list)