Add VictoriaLogs for centralized journald across all hosts

control runs VictoriaLogs (:9428, 30d, mesh-scoped) with a matching
Grafana datasource. Each host ships journald via systemd's own
journald.upload to the /insert/journald endpoint -- no extra agent.
control uploads over loopback so its logs survive a mesh outage; ns1
and ns2 push over the mesh.

modules/monitoring/server.nix

@@ -12,6 +12,7 @@ let
   mesh = import ../mesh-hosts.nix { inherit config lib; };
   probes = import ./blackbox-probes.nix { inherit lib; };
   vmPort = 8428;
+  logsPort = 9428;
   grafanaPort = 3000;
   controlV6 = mesh.hosts.control;
 
@@ -60,6 +61,17 @@ in
     };
   };
 
+  # Centralized logs: VictoriaLogs ingests journald from all three hosts, each
+  # of which runs systemd-journal-upload against /insert/journald (exporters.nix).
+  # Binds all interfaces because ns1/ns2 push over the mesh; the firewall rule at
+  # the bottom scopes 9428 to the mesh subnet and the Hetzner firewall closes the
+  # public side. Retention is set via extraOptions (no dedicated NixOS option).
+  services.victorialogs = {
+    enable = true;
+    listenAddress = ":${toString logsPort}";
+    extraOptions = [ "-retentionPeriod=30d" ];
+  };
+
   # Admin password generated once and stored as a clan secret. Retrieve with:
   #   clan vars get control grafana-admin/password
   clan.core.vars.generators.grafana-admin = {
@@ -76,6 +88,9 @@ in
 
   services.grafana = {
     enable = true;
+    # VictoriaLogs datasource plugin so journald is greppable from Grafana,
+    # alongside the metrics datasource.
+    declarativePlugins = [ pkgs.grafanaPlugins.victoriametrics-logs-datasource ];
     settings = {
       server = {
         http_addr = "::";
@@ -102,6 +117,13 @@ in
             url = "http://127.0.0.1:${toString vmPort}";
             isDefault = true;
           }
+          {
+            name = "VictoriaLogs";
+            type = "victoriametrics-logs-datasource";
+            uid = "victorialogs";
+            access = "proxy";
+            url = "http://127.0.0.1:${toString logsPort}";
+          }
         ];
       };
       dashboards.settings = {
@@ -117,8 +139,9 @@ in
     };
   };
 
-  # Grafana reachable only from the ZeroTier mesh (admin laptops + servers).
+  # Grafana (admin laptops + servers) and VictoriaLogs ingestion (ns1/ns2 push
+  # journald over the mesh) reachable only from the ZeroTier mesh.
   networking.firewall.extraInputRules = ''
-    ip6 saddr ${mesh.subnet} tcp dport ${toString grafanaPort} accept
+    ip6 saddr ${mesh.subnet} tcp dport { ${toString grafanaPort}, ${toString logsPort} } accept
   '';
 }