Add possibility to keep communication between the main app and Hosted Instances internal (within cluster)

### Description

When deployed on Kubernetes, Hosted Instances running in a project namespace make server-side requests (such as authentication operations) back to the main app; they currently route through the main app's public hostname.

Routing internal traffic out to public endpoints creates unnecessary external network dependencies and causes issues when public endpoints are protected or restricted by perimeter access controls, as internal workloads cannot route through those external barriers.

The goal of this issue is to identify and implement a solution that will keep the internal communication between Hosted Instances and the Forge app, private.

### Which customers would this be available to

Everyone - CE/Starter/Team/Enterprise

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

I have provided an initial effort estimate

No files or tests are named. Start by tracing how Hosted Instances construct server-side requests to the Forge app and reviewing the Kubernetes deployment and networking configuration; done means those requests remain private within the cluster without relying on the public hostname.

Written by the indexing model from the issue text.