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.

索引モデルが issue の本文から書いたものです。