fix: reuse upload credentials for device config sync

- load server sync creds from db/upload_creds.env like uploads do
- fall back from YARIS_DEVICE_KEY to existing YARIS_API_KEY
- document shared credential flow for server-synced identity
- add test coverage for sync credential fallback

Co-authored-by: Copilot <[email protected]>

tests/test_app_device_config_runtime.py

@@ -1,4 +1,5 @@
 import unittest
+import os
 from unittest.mock import Mock, patch
 
 from modules import app_account, app_car, app_device_sync
@@ -55,6 +56,13 @@ class DeviceSyncReloadCallbackTest(unittest.TestCase):
         finally:
             app_device_sync._RELOAD_CALLBACK = original
 
+    @patch.dict(os.environ, {"YARIS_SERVER_URL": "https://example.test", "YARIS_API_KEY": "upload-key"}, clear=True)
+    def test_sync_credentials_fall_back_to_upload_api_key(self):
+        server_url, device_key = app_device_sync._server_credentials()
+
+        self.assertEqual(server_url, "https://example.test")
+        self.assertEqual(device_key, "upload-key")
+
 
 if __name__ == "__main__":
     unittest.main()