[Feature Request]: Admins should enforce the allowable payment plan for agents to choose from

### 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

Feature request from users in allow defining payment plans on MPM web platform that control what agents can use on the the agent app. There could be a risk of agents selecting the wrong payment plan.

MPM currently does not support this feature.

### Proposed Solution

Implement a feature that allows creating allowable plan that can be set for an agent. It should be editable and flexible.

### Alternatives Considered

N/A

### Additional Information

_No response_

Start by reviewing the existing payment-plan and agent-management flows in the MPM web platform. Clarify how allowable plans are created, assigned, edited, and enforced, then define acceptance criteria covering agent plan selection and administrator control.

Written by the indexing model from the issue text.