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/docker_health_check.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# docker_health_check.sh — detect unhealthy Docker containers.
+set -euo pipefail
+
+if ! command -v docker >/dev/null 2>&1; then
+  echo '{"health":"healthy"}'
+  exit 0
+fi
+
+UNHEALTHY=$(docker ps --filter "health=unhealthy" --format "{{.Names}}" 2>/dev/null || true)
+
+if [ -n "$UNHEALTHY" ]; then
+  COUNT=$(echo "$UNHEALTHY" | wc -l | tr -d ' ')
+  NAMES=$(echo "$UNHEALTHY" | tr '\n' ',' | sed 's/,$//')
+  echo "{\"health\":\"degraded\",\"signalKind\":\"docker-unhealthy\",\"evidence\":\"$COUNT unhealthy container(s): $NAMES\",\"metrics\":{\"docker_unhealthy\":$COUNT}}"
+else
+  TOTAL=$(docker ps -q 2>/dev/null | wc -l | tr -d ' ' || echo 0)
+  echo "{\"health\":\"healthy\",\"metrics\":{\"docker_unhealthy\":0,\"docker_total\":$TOTAL}}"
+fi