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_usage_check.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+# disk_usage_check.sh — disk usage and inode usage per mountpoint.
+set -euo pipefail
+
+MOUNTS=$(df -k 2>/dev/null | awk 'NR>1 && $1 ~ /^\// && $NF !~ /^\/(snap|dev|proc|sys|run|private)/ {print $NF}' || true)
+FIRST=1
+
+echo -n '{"health":"healthy","metrics":{'
+for m in $MOUNTS; do
+  LINE=$(df -k "$m" 2>/dev/null | awk 'NR==2 {print $3, $4, $5, $7}' | tr -d '%' || true)
+  if [ -z "$LINE" ]; then continue; fi
+  USED=$(echo "$LINE" | awk '{print $1}')
+  FREE=$(echo "$LINE" | awk '{print $2}')
+  PCT=$(echo "$LINE" | awk '{print $3}')
+
+  INODE_LINE=$(df -i "$m" 2>/dev/null | awk 'NR==2 {print $5}' | tr -d '%' || echo "0")
+  INODE_PCT="${INODE_LINE:-0}"
+
+  KEY=$(echo "$m" | sed 's|/|_|g' | sed 's|^_||')
+  [ -z "$KEY" ] && KEY="root"
+
+  if [ $FIRST -eq 0 ]; then echo -n ','; fi
+  FIRST=0
+  echo -n "\"disk_${KEY}_pct\":$PCT,\"inode_${KEY}_pct\":$INODE_PCT"
+done
+echo '}}'