Replace shared redis-exporter VMPodScrape with per-app scrapes

Per review: drop the cross-namespace label-selector approach. Each app now
carries its own VMPodScrape next to its redis/valkey manifests, selecting on
the deployment's existing pod labels (mirroring the arrstack exportarr
pattern), so no opt-in monitoring.unkin.net/redis-exporter label is needed.

- Remove the monitoring.unkin.net/redis-exporter pod label from the
  artifactapi/authentik/litellm redis and gitea/netbox valkey deployments
- Delete apps/base/observability/vmpodscrape-redis.yaml
- Add a local-namespace VMPodScrape (port metrics, path /metrics) to
  artifactapi, authentik, litellm (app=redis), gitea (app=gitea-valkey),
  netbox (app=netbox-valkey), and jellyfin (valkey-operator node labels)

apps/base/artifactapi/kustomization.yaml

@@ -18,4 +18,5 @@ resources:
   - ui-hpa.yaml
   - vaultauth.yaml
   - vaultstaticsecret.yaml
+  - vmpodscrape.yaml
   - vpa.yaml