refactor: align Bitstream checkSum with upstream ChecksumInfo

Match DSpace/dspace-angular@main (commit 4f72074): use the exported
ChecksumInfo interface and `checkSum: ChecksumInfo` positioned after
`description`, replacing the inline anonymous type, to minimize conflicts on a
future DSpace 9 migration. Display and i18n key are unchanged (upstream renders
no checksum row in the full item view).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jr-rk

src/app/core/shared/bitstream.model.ts

@@ -21,6 +21,11 @@ import { ChildHALResource } from './child-hal-resource.model';
 import { DSpaceObject } from './dspace-object.model';
 import { HALLink } from './hal-link.model';
 
+export interface ChecksumInfo {
+  checkSumAlgorithm: string;
+  value: string;
+}
+
 @typedObject
 @inheritSerialization(DSpaceObject)
 export class Bitstream extends DSpaceObject implements ChildHALResource {
@@ -33,19 +38,16 @@ export class Bitstream extends DSpaceObject implements ChildHALResource {
   sizeBytes: number;
 
   /**
-   * The checksum of this Bitstream, including the algorithm used to calculate it
+   * The description of this Bitstream
    */
   @autoserialize
-  checkSum: {
-    checkSumAlgorithm: string;
-    value: string;
-  };
+  description: string;
 
   /**
-   * The description of this Bitstream
+   * The checksum information of this Bitstream
    */
   @autoserialize
-  description: string;
+  checkSum: ChecksumInfo;
 
   /**
    * The name of the Bundle this Bitstream is part of