[Feature Request]: Reassign a village to a different mini-grid

### 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

If a village is assigned to the wrong mini-grid, there is no way to move it without recreating it.

### Proposed Solution

Allow moving a village from one mini-grid to another while preserving its associated data.

### Alternatives Considered

N/A

### Additional Information

_No response_

No files, tests, or entry points are named. Start by locating the village and mini-grid models and the existing village-assignment flow, then trace how associated data is linked. Done means a village can be moved between mini-grids without recreating it or losing its associated data.

Written by the indexing model from the issue text.