[Feature Request]: Consolidate `TicketUser`, `MaintenanceUser` and `Agents`

### Preflight Checklist

- [x] I have read the [Contributing Guidelines](https://github.com/EnAccess/micropowermanager/blob/main/CONTRIBUTING.md) for this project, if it exists.
- [x] I agree to follow the [Code of Conduct](https://github.com/EnAccess/micropowermanager/blob/main/CODE_OF_CONDUCT.md) that this project adheres to.
- [x] I have searched the [issue tracker](https://github.com/EnAccess/micropowermanager/issues) for a feature request that matches the one I want to file, without success.

### Problem Description

Currently the MicroPowerManager know many different Model related to "persons" that can interact with Tickets...

Namely `TicketUser`, `MaintenanceUser` and `Agents`. This is way too confusing and adds unneeded complexity.

Also, it's not immediately clear what the "actual" difference between these is (in terms in permissions).

### Proposed Solution

Please consolidate these into a single object. Probably `Agent`.

### Alternatives Considered

N/A

### Additional Information

_No response_