scheduler: add ping + ssh-script check kinds, metrics refactor, 17 host check scripts

- Refactor executeCheck to return checkResult struct with metrics map
- Add ping check kind (ICMP reachability via system ping, macOS+Linux)
- Add ssh-script check kind (remote host exec via SSH, allowlisted scripts)
- Add threshold evaluation (warn/crit per metric from check config JSONB)
- Add inode tracking to disk check
- All 4 existing checks now return structured metrics
- 17 check scripts: cpu, memory, load, swap, disk_usage, disk_smart,
  updates, zfs, process, uptime, oom, journal, time, fd, docker_health,
  caddy_error_rate, backup_freshness
- Auto-deploy via tools/setup-checks.sh -> checks/install.sh on git pull
- Add ping to OpenAPI CheckKind enum and generated Go types

checks/updates_check.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# updates_check.sh — pending apt security updates and reboot-required flag.
+set -euo pipefail
+
+SECURITY=0
+REBOOT=0
+
+if command -v apt >/dev/null 2>&1; then
+  apt update -qq >/dev/null 2>&1 || true
+  SECURITY=$(apt list --upgradable 2>/dev/null | grep -c '\-security' || true)
+fi
+
+if [ -f /var/run/reboot-required ]; then
+  REBOOT=1
+fi
+
+echo "{\"health\":\"healthy\",\"metrics\":{\"security_updates\":$SECURITY,\"reboot_required\":$REBOOT}}"