`ArbitraryFileIdManager.index()` does not return the existing file ID if present

## Description

The implementation of the `index()` method always creates a new file ID, even if one already exists:

```py
def index(self, path: str) -> str:
    # create new record
    with self.con:
        id = self._create(path)
        return id
```

It should check `get_id()` first and return it if it exists.

