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

hosts/unstable/hosea/default.nix

@@ -21,6 +21,11 @@ in
     ./hardware-configuration.nix
   ];
 
+  age.secrets.grafana-secret-key = {
+    file = ../../../secrets/grafana-secret-key.age;
+    owner = "grafana";
+  };
+
   # Bootloader
   boot = {
     loader = {
@@ -124,7 +129,7 @@ in
         ];
       };
       settings = {
-        security.secret_key = "123456789";
+        security.secret_key = "$__file{${config.age.secrets.grafana-secret-key.path}}";
         server = {
           domain = "${config.networking.hostName}.shire-zebra.ts.net";
           enforce_domain = true;