Add jellyfin (HA fork) app under a new media project

Deploys the jellyfin-ha fork (git.unkin.net/unkin/jellyfin-ha) to au-syd1
via ArgoCD, under a dedicated media AppProject/ApplicationSet rather than
extending platform.

- New media AppProject + media-apps ApplicationSet (watches
  apps/overlays/*/jellyfin); registered in the argocd kustomizations
- apps/base/jellyfin: namespace, deployment (single replica to start),
  service, in-namespace Redis (transcode session store), gateway + httproute
  at jellyfin.k8s.syd1.au.unkin.net
- Storage: RWO config (cephrbd), RWX transcode scratch and RWX media
  library (cephfs) per the HA fork's pod-takeover requirement
- au-syd1 overlay

apps/base/jellyfin/namespace.yaml

@@ -0,0 +1,5 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: jellyfin