feat(api): Add `Image` type for built-in cover property

## Description
Add a dedicated `Image` GraphQL type with additional dimension information for the built-in `cover` property.

## Implementation
```graphql
type Image {
url: String!
width: Int
height: Int
fileType: String
}

type Entity {
# (Change) cover from `String` to `Image`
cover: Image

# ...other attribute
}
```

Find the GraphQL schema definitions in the codebase, likely in a file like `schema.graphql` or within Rust modules using a GraphQL library. Locate the `Entity` type and change its `cover` field from `String` to the new `Image` type as defined. Ensure the `Image` type is added to the schema with the specified fields. Test the change by running any existing GraphQL tests or queries to verify the new type works.

Written by the indexing model from the issue text.