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/disk_smart_check.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+# disk_smart_check.sh — SMART pre-failure indicators for physical disks.
+set -euo pipefail
+
+if ! command -v smartctl >/dev/null 2>&1; then
+  echo '{"health":"healthy"}'
+  exit 0
+fi
+
+DISKS=$(lsblk -ndo NAME,TYPE 2>/dev/null | awk '$2=="disk"{print "/dev/"$1}' || true)
+if [ -z "$DISKS" ]; then
+  echo '{"health":"healthy"}'
+  exit 0
+fi
+
+FAILED=""
+for dev in $DISKS; do
+  INFO=$(smartctl -H "$dev" 2>/dev/null || true)
+  if ! echo "$INFO" | grep -q "PASSED\|OK"; then
+    MODEL=$(smartctl -i "$dev" 2>/dev/null | awk -F': ' '/Device Model|Product/{print $2; exit}' || echo "$dev")
+    FAILED="$FAILED $MODEL"
+  fi
+done
+
+if [ -n "$FAILED" ]; then
+  echo "{\"health\":\"degraded\",\"signalKind\":\"disk-smart-fail\",\"evidence\":\"SMART check failed for:$(echo "$FAILED" | sed 's/ /, /g')\"}"
+else
+  echo '{"health":"healthy"}'
+fi