[API] Add access to individual survey results data via entities API

Current sample request:

```
{
entities(ids: "aspect_ratio") {
id
appearsIn {
as
question {
id
}
edition {
id
}
section {
id
}
survey {
id
}
}
}
}
```

- Add a way to get specific data

Start by tracing the entities API behind the sample GraphQL request and documenting how entity data is currently resolved. Clarify which individual survey-results data and query shape are required; done means the agreed data is accessible through the entities API with coverage for the requested behavior.

Written by the indexing model from the issue text.