V6.1 continue refining backup: switch to Synology Drive Client for remote backup

tests/ui/test_runtime_settings_store.py

@@ -83,3 +83,18 @@ def test_save_runtime_settings_rejects_invalid_values(tmp_path: Path):
             updates={"port": "not-a-number"},
             env_file_path=env_path,
         )
+
+
+def test_runtime_settings_uses_override_env_file(tmp_path: Path, monkeypatch):
+    settings = _settings_for_runtime_editing(tmp_path)
+    target = tmp_path / ".env.production"
+    target.write_text("OPENROUTER_API_KEY=test-key\nPORT=8000\n", encoding="utf-8")
+    monkeypatch.setenv("RUNTIME_SETTINGS_ENV_FILE", str(target))
+
+    snapshot = save_runtime_settings(
+        settings=settings,
+        updates={"port": "9001"},
+    )
+
+    assert snapshot.env_file_path == target
+    assert "PORT=9001" in target.read_text(encoding="utf-8")