arrstack: sonarr/radarr/prowlarr/nzbget on shared CephFS media (#366)

## Why
Stands up the download-automation stack (sonarr, radarr, prowlarr, nzbget) in a new `arrstack` namespace, sharing the same CephFS TV/movies subvolumes that jellyfin serves read-only (jellyfin static-PV rework is on the base branch, PR #237). Downloads and libraries live on one filesystem per media type so the arrs import with atomic hardlink moves.

## Changes
- Add `apps/base/arrstack/` namespace with its own static CephFS PVs (`arrstack-media-tv`, `arrstack-media-movies`) + RWX PVCs (`media-tv`, `media-movies`) bound to the same rootPaths jellyfin uses; each namespace gets a unique PV name/volumeHandle pinned by claimRef.
- Deploy sonarr (mounts media-tv RW), radarr (media-movies RW), prowlarr (config only), nzbget (both media PVCs RW); single replica, Recreate strategy, LinuxServer images via artifactapi dockerhub remote, PUID/PGID/TZ, probes, fsGroup for CephFS writability.
- Per-app config PVCs on `cephrbd-fast-retain` (RWO, 5Gi) for the SQLite state; internal-only Gateway + HTTPRoute per app (traefik-internal, vault-issuer, external-dns `<app>.k8s.syd1.au.unkin.net`), no oauth yet.
- nzbget: initContainer seeds `/config/nzbget.conf` (copy image template if absent, append managed path/category block once) and creates the media dirs; category `tv` -> `/media/tv/downloads`, `movies` -> `/media/movies/downloads`.
- Wire ArgoCD: add `apps/overlays/*/arrstack` to the media ApplicationSet. (media AppProject already gains the arrstack destination + PersistentVolume whitelist on the base branch.)

## Depends on
terraform-artifactapi PR #31 (allowlist `^linuxserver/` on the dockerhub remote) must be merged + applied before these pods can pull.

## Follow-up (terraform)
Root folders (/media/tv/series, /media/movies/films), download-client wiring (host `nzbget`, port 6789, categories tv/movies) and API keys are configured later via terraform-arr.

---------

Co-authored-by: Ben Vincent <neotheo@gmail.com>
Co-authored-by: Ben Vin <neotheo@gmail.com>
Reviewed-on: #366
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

apps/base/arrstack/prowlarr/deployment.yaml

@@ -0,0 +1,67 @@
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: prowlarr
+  namespace: arrstack
+spec:
+  replicas: 1
+  strategy:
+    # RWO config PVC + single stateful SQLite DB: never run two pods at once.
+    type: Recreate
+  selector:
+    matchLabels:
+      app: prowlarr
+  template:
+    metadata:
+      labels:
+        app: prowlarr
+    spec:
+      securityContext:
+        fsGroup: 1000
+        fsGroupChangePolicy: OnRootMismatch
+      containers:
+        - name: prowlarr
+          image: artifactapi.k8s.syd1.au.unkin.net/dockerhub/linuxserver/prowlarr:2.5.2
+          imagePullPolicy: IfNotPresent
+          ports:
+            - name: http
+              containerPort: 9696
+              protocol: TCP
+          env:
+            - name: PUID
+              value: "1000"
+            - name: PGID
+              value: "1000"
+            - name: TZ
+              value: Australia/Sydney
+          livenessProbe:
+            httpGet:
+              path: /ping
+              port: http
+            initialDelaySeconds: 30
+            periodSeconds: 30
+            timeoutSeconds: 5
+            failureThreshold: 3
+          readinessProbe:
+            httpGet:
+              path: /ping
+              port: http
+            initialDelaySeconds: 10
+            periodSeconds: 10
+            timeoutSeconds: 5
+            failureThreshold: 3
+          resources:
+            requests:
+              cpu: 100m
+              memory: 256Mi
+            limits:
+              cpu: "1"
+              memory: 512Mi
+          volumeMounts:
+            - name: config
+              mountPath: /config
+      volumes:
+        - name: config
+          persistentVolumeClaim:
+            claimName: prowlarr-config