Enable peer-to-peer failover protection

### Epic

_No response_

### Description

**As a user with critical flows running on edge devices**

**I want to designate a failover system**

**So that my operations are protected from an instance failing**

One approach is that FlowFuse (specifically the Device Agent?) would allow device groups to select a 'backup' instance that turns on when another instance fails.

### Which customers would this be available to

None

### Acceptance Criteria

- [ ] criteria 1
- [ ] criteria 2
- [ ] criteria 3
- ...

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

No files, tests, or entry points are identified. Start by clarifying the failover behavior, device-group configuration, failure detection, and acceptance criteria with the maintainers; the issue is done only when those requirements and validation steps are defined and implemented.

Written by the indexing model from the issue text.