Moliri API v1: designed as a standard

The new Node backend gets an API designed from scratch instead of copying the old PHP endpoints. Errors always come in one format with real status codes. Field values are keyed by field id, so renaming a field breaks nothing. When two people save the same field, the second one sees both values instead of silently losing one. Long work such as exports, OCR and uploads runs as a job whose progress and failures the app can see.