arrstack: add kids backend tier (sonarr/radarr/nzbget-kids)

Split the arrstack media pipeline into an adult and a kids tier so kids
content lives in its own libraries, databases, and downloader while
reusing the shared prowlarr, CNPG cluster, Valkey, media PVCs, and S3
buckets. Backend workloads only — no routing/ingress or arrproxy changes
in this PR; the adult instances are untouched.

- sonarr-kids/radarr-kids: mirror the adult -unkin fork Deployments
  (same images/tags, 3 active-active replicas), pointed at their own
  Postgres DBs (sonarr-kids-main/radarr-kids-main), UrlBase
  /3df803/sonarr and /3df803/radarr, and kids MediaCover/Backup S3
  prefixes; media mounts use the tvshows/kids and movies/kids subPaths
  of the shared media-tv/media-movies PVCs.
- nzbget-kids: second downloader on its own RWO config PVC, writing
  completed downloads into the kids media subtrees for same-filesystem
  hardlink imports.
- postgres: two managed roles + Database CRDs (sonarr-kids/radarr-kids)
  and their VSO-synced <app>-kids-db credential secrets.
- per-app VMPodScrape for each kids exportarr sidecar; register the
  three new apps in the base kustomization.

apps/base/arrstack/postgres/kustomization.yaml

@@ -9,3 +9,5 @@ resources:
   - database-sonarr.yaml
   - database-radarr.yaml
   - database-prowlarr.yaml
+  - database-sonarr-kids.yaml
+  - database-radarr-kids.yaml