refactor: rename duplicate AdminRole interface, type sellerId as UUID

AdminRole was defined twice with unrelated shapes (core/auth's real
JWT role union vs. the Users admin page's display interface), flagged
in BACKEND-API-REFERENCE.md \u00a72b as needing a rename. Renamed the
Users-page one to AdminUserRoleRecord.

sellerId was bare string in admin-order/admin-product/item models
while core/sellers/models/seller-scope.model.ts already used the
shared UUID alias. Aligned all three to UUID for consistency (UUID is
currently just = string, so this is a documentation-level type change,
not a behavior change).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/models/item.model.ts

@@ -1,3 +1,5 @@
+import { UUID } from '../shared/types/primitive.types';
+
 interface Photo {
   photo?: string;
   video?: string;
@@ -182,7 +184,7 @@ export interface Item {
    * Absent means marketplace-owned, exactly like every product today -
    * nothing reads this field yet, nothing breaks by it being undefined.
    */
-  sellerId?: string;
+  sellerId?: UUID;
 }
 
 export interface CartItem extends Item {