Horizontal scaling beyond 2 instances for High Availability mode

### Description

Currently FlowFuse High Availability supports up to 2 total replica instances of Node-RED in HA mode.

It would be helpful to have the ability to expand to a larger number of replicas if expanded horizontal capacity is needed.

### Which customers would this be available to

Enterprise Tier Only (EE)

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

I have provided an initial effort estimate

**Requested by:**

https://app-eu1.hubspot.com/contacts/26586079/record/0-2/10975729652

The issue does not identify files, tests, or an implementation entry point. Start by locating the High Availability replica-limit logic, then determine the design and validation needed to support more than two Node-RED replicas for Enterprise users.

Written by the indexing model from the issue text.