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/zfs_check.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+# zfs_check.sh — ZFS pool health.
+set -euo pipefail
+
+if ! command -v zpool >/dev/null 2>&1; then
+  echo '{"health":"healthy"}'
+  exit 0
+fi
+
+STATUS=$(zpool status -x 2>&1 || true)
+SCRUB_OVERDUE=""
+
+if echo "$STATUS" | grep -q "all pools are healthy"; then
+  for pool in $(zpool list -Ho name 2>/dev/null || true); do
+    LAST=$(zpool status "$pool" 2>/dev/null | awk '/scan:/{print $0}' || true)
+    if [ -z "$LAST" ] || echo "$LAST" | grep -q "scrub repaired"; then
+      SCRUB_OVERDUE="$pool"
+      break
+    fi
+  done
+  if [ -n "$SCRUB_OVERDUE" ]; then
+    echo "{\"health\":\"degraded\",\"signalKind\":\"zfs-scrub-overdue\",\"evidence\":\"pool $SCRUB_OVERDUE scrub has errors or is overdue\"}"
+  else
+    echo '{"health":"healthy"}'
+  fi
+else
+  HEALTH=$(echo "$STATUS" | grep "state:" | awk '{print $2}' | head -1)
+  echo "{\"health\":\"degraded\",\"signalKind\":\"zfs-degraded\",\"evidence\":\"pool state: $HEALTH\"}"
+fi