Add identifier ID and ContentType to `/api/v1/admin/transactions` GET list response

[sanaderi]

The /api/v1/admin/transactions GET endpoint should be updated to include additional fields in the list response only. These fields are required for better visibility and debugging in admin transaction listings, without changing the core behavior of the endpoint.

Requirements

• Add identifierId (or identifier_id) to each transaction item in the GET list response
• Add contentType field to each transaction item in the GET list response
• Ensure values are correctly mapped from existing domain/database fields
• Do not introduce breaking changes to existing consumers
• Maintain current filtering, sorting, and pagination behavior unchanged

Expected Outcome

• Each transaction in /api/v1/admin/transactions list response includes:
  • Identifier ID
  • ContentType
• Existing API behavior remains unchanged except for additional response fields
• Admin UI / consumers can display richer transaction metadata

Notes

• Ensure consistent naming conventions (camelCase vs snake_case)
• Verify whether contentType is already available in entity or needs mapping
• Confirm no performance impact on list queries (especially with joins or computed fields)