[User Story] Delete an account from the service

**As a** system administrator  
**I need** to delete accounts  
**So that** outdated records can be cleaned up  

### Details and Assumptions
* DELETE endpoint: `/accounts/<id>`  
* Irreversible deletion  

### Acceptance Criteria
```gherkin
Given a valid account ID  
When a DELETE request is sent  
Then the account is removed from the database