feat: add multi-device provisioning and local remote-config UI

- add multi-device portal device management, key rotation, and sync request APIs
- add Pi settings UI for remote server URL, device name, and API key
- add local test-connection and sync-now actions
- make upload and config sync reuse dynamic shared credentials
- add Phase 25.5c migration and docs updates

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

server/modules/device_config.py

@@ -30,6 +30,8 @@ def update_device_config(device_id, data):
     Validates input and updates config_version on change.
     """
     errors = {}
+    if db.get_device_by_id(device_id) is None:
+        return False, {"error": "Device not found."}
     
     if not isinstance(data, dict):
         return False, {"error": "Invalid request body"}
@@ -87,4 +89,3 @@ def update_device_config(device_id, data):
             )
     
     return True, {}
-