monitoring02: add VictoriaMetrics, vmalert, and Alertmanager

Set up the core metrics stack on monitoring02 as Phase 2 of the
monitoring migration. VictoriaMetrics replaces Prometheus with
identical scrape configs (22 jobs including auto-generated targets).

- VictoriaMetrics with 3-month retention and all scrape configs
- vmalert evaluating existing rules.yml (notifier disabled)
- Alertmanager with same routing config (no alerts during parallel op)
- Grafana datasources updated: local VictoriaMetrics as default
- Static user override for credential file access (OpenBao, Apiary)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hosts/monitoring02/default.nix

@@ -2,5 +2,6 @@
   imports = [
     ./configuration.nix
     ../../services/grafana
+    ../../services/victoriametrics
   ];
 }