feat: comprehensive observability stack
- Add restic_backups and mattermost scrape jobs to Prometheus config
- Add UnPoller deployment+service+secret for UniFi metrics (unpoller.yaml)
- Add unifi scrape job targeting unpoller in-cluster
- Add 4 Grafana dashboards as NixOS-managed etc files on hosea:
- system-health: CPU, memory, disk, network per host
- backup-health: restic backup freshness and duration
- kubernetes: pod counts, restart rates, node resources
- network: DNS queries, DHCP leases, UniFi ports, ping latency
- Add grafana-api-token agenix secret placeholder on hosea
- Open firewall ports 9100/9427/9558 for prometheus exporters in baseline.nix