monitoring: auto-generate Prometheus scrape targets from host configs

Add homelab.monitoring NixOS options (enable, scrapeTargets) following
the same pattern as homelab.dns. Prometheus scrape configs are now
auto-generated from flake host configurations and external targets,
replacing hardcoded target lists.

Also cleans up alert rules: snake_case naming, fix zigbee2mqtt typo,
remove duplicate pushgateway alert, add for clauses to monitoring_rules,
remove hardcoded WireGuard public key, and add new alerts for
certificates, proxmox, caddy, smartctl temperature, filesystem
prediction, systemd state, file descriptors, and host reboots.

Fixes grafana scrape target port from 3100 to 3000.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/monitoring/external-targets.nix

@@ -0,0 +1,12 @@
+# Monitoring targets for hosts not managed by this flake
+# These are manually maintained and combined with auto-generated targets
+{
+  nodeExporter = [
+    "gunter.home.2rjus.net:9100"
+  ];
+  scrapeConfigs = [
+    { job_name = "smartctl"; targets = [ "gunter.home.2rjus.net:9633" ]; }
+    { job_name = "ghettoptt"; targets = [ "gunter.home.2rjus.net:8989" ]; }
+    { job_name = "restic_rest"; targets = [ "10.69.12.52:8000" ]; }
+  ];
+}