Add API endpoint for SSH keypair reset/revocation

## Background

Currently there is no backend API to reset or revoke a user's registered SSH keypair. Users who have registered an SSH keypair in User Settings have no way to reset it to a new keypair or revoke it through the WebUI.

## Task

Implement a backend API endpoint that allows:

1. Resetting the user's registered SSH keypair (generate a new one)
1. Revoking / removing the registered keypair (fall back to per-session dropbear keys)

## Context

- This is required for the WebUI SSH keypair management UX improvements (see related FR issue)
- Assignee suggestion: HyeokJin Kim (per Teams discussion)
- Once this API is available, the WebUI will implement the corresponding reset UI

## References

- Teams discussion: UX Consistency improvements for SSH keypairs
- Captured while working on branch: claude/upbeat-keller

JIRA Issue: BA-5398