Support multi-tenancy in the Chat UI

[nstogner]

Currently we have authentication explicitly disabled and are missing data persistence (needed for tracking users) on Open WebUI. This means that any user can view chat threads. Also Pod restarts cause any thread history to be lost.

Short term it probably makes sense to expose a way to enable user logins and add a PVC to Open WebUI. I don't think we want KubeAI to rely on dependencies like NFS storage so we might want to switch the OpenWeb UI Pod to be managed by a StatefulSet and set replicas: 1 (& a replacement upgrade strategy) with a standard PVC today.

Longer term, we need to consider how we are going to tackle AuthN/AuthZ in a consistent way across all UIs for KubeAI (assuming we will be adding a platform UI for building assistants, etc).