fix(monitoring): order smartctl exporter after network-online.target (#372)

The exporter binds to the host LAN address, which systemd-networkd
assigns during boot. Without ordering, the service starts before the
address exists, `bind: cannot assign requested address` trips the
start limit, and the unit stays failed until restarted manually.

Order the unit after network-online.target, and since wait-online is
best-effort (120s timeout), also retry patiently so the unit self-heals
when the link comes up late.

modules/nixos/base/monitoring.nix

@@ -54,4 +54,17 @@
     port = 9633;
     maxInterval = "60s";
   };
+
+  # The exporter binds to the LAN address, which networkd assigns during boot;
+  # without ordering it races the address and trips the start limit (seen on ai).
+  # wait-online is best-effort (120s timeout), so also retry patiently to
+  # self-heal when the link comes up late.
+  systemd.services.prometheus-smartctl-exporter = lib.mkIf config.modules.hardwareTools.enable {
+    after = [ "network-online.target" ];
+    wants = [ "network-online.target" ];
+    serviceConfig = {
+      RestartSec = "10s";
+      StartLimitIntervalSec = 0;
+    };
+  };
 }