Controllers not returning ReponseEntity with status

The controllers are returning String but not responseEntity with status.
Also returning a json would be better for serializable vs parcelable property.

No files or tests are named. Start by locating the controller classes and their endpoints that currently return String, then review the existing API response conventions. Done means the affected endpoints return ResponseEntity values with appropriate statuses and JSON representations, with response behavior covered by tests if the project has them.

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