fix: security and monitoring improvements

Issue 1 - Grafana secret key:
- Replace hardcoded '123456789' secret_key in hosea Grafana config
  with an agenix-managed secret ($__file interpolation)
- Add age.secrets.grafana-secret-key declaration to hosea config
- Register grafana-secret-key.age in secrets/secrets.nix (publicKeys=everyone)
- NOTE: The .age file still needs to be created by running:
    agenix -e secrets/grafana-secret-key.age
  and entering a strong random secret (e.g. from 'pwgen -s 64 1')

Issue 2 - Prometheus stale targets:
- Remove retired vm-gitlab host from all three scrape jobs
  (node, systemd, ping) in manifests/monitoring/config.yaml
- linode was already present in all scrape jobs

Issue 3 - Prometheus alerting gaps:
- Wire up rule_files in prometheus.yml pointing at alerts.yml
- Add manifests/monitoring/alerts.yaml with alerting rules:
    * HostDown (node unreachable > 2m) - critical
    * HighCPULoad (>90% for 10m) - warning
    * LowDiskSpace (<10% free) - warning
    * CriticalDiskSpace (<5% free) - critical
    * HighMemoryUsage (>90% for 10m) - warning
    * ResticBackupStaleness (no success > 26h) - warning
    * PodCrashLooping - warning
    * PodNotReady (>10m) - warning
- Mount alerts ConfigMap as separate volume subPath in deployment
  so prometheus.yml and alerts.yml are both accessible at
  /etc/prometheus/ without conflicts

secrets/secrets.nix

@@ -90,6 +90,8 @@ in
   "restic-env.age".publicKeys = everyone;
   "restic-pw.age".publicKeys = everyone;
 
+  "grafana-secret-key.age".publicKeys = everyone;
+
   "dendrite.age".publicKeys = everyone;
   "dendrite_key.age".publicKeys = everyone;