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/process_check.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# process_check.sh — systemd service liveness.
+set -euo pipefail
+
+SERVICE="${1:-}"
+if [ -z "$SERVICE" ]; then
+  echo '{"health":"unknown","signalKind":"process-check","evidence":"no service name provided"}'
+  exit 0
+fi
+
+if ! command -v systemctl >/dev/null 2>&1; then
+  echo '{"health":"unknown","signalKind":"process-check","evidence":"systemctl not found"}'
+  exit 0
+fi
+
+STATE=$(systemctl is-active "$SERVICE" 2>/dev/null || echo "unknown")
+
+if [ "$STATE" = "active" ]; then
+  echo "{\"health\":\"healthy\"}"
+else
+  echo "{\"health\":\"degraded\",\"signalKind\":\"$SERVICE\",\"evidence\":\"$SERVICE is $STATE\"}"
+fi