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/caddy_error_rate.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# caddy_error_rate.sh — 5xx error rate from Caddy JSON access logs.
+set -euo pipefail
+
+LOG_DIR=""
+if [ -d /var/log/caddy ]; then
+  LOG_DIR="/var/log/caddy"
+elif [ -d /var/lib/caddy/logs ]; then
+  LOG_DIR="/var/lib/caddy/logs"
+elif [ -d /opt/caddy/logs ]; then
+  LOG_DIR="/opt/caddy/logs"
+fi
+
+if [ -z "$LOG_DIR" ]; then
+  echo '{"health":"healthy"}'
+  exit 0
+fi
+
+CUTOFF=$(date -u -d "5 minutes ago" +%Y-%m-%dT%H:%M 2>/dev/null || date -u -v-5M +%Y-%m-%dT%H:%M 2>/dev/null || echo "")
+
+TOTAL=0
+ERR_5XX=0
+
+for LOG in "$LOG_DIR"/access*.log "$LOG_DIR"/access*.json "$LOG_DIR"/*.log 2>/dev/null; do
+  [ -f "$LOG" ] || continue
+  [ -r "$LOG" ] || continue
+
+  if [ -n "$CUTOFF" ]; then
+    NEW=$(awk -v cutoff="$CUTOFF" '$0 >= cutoff {print}' "$LOG" 2>/dev/null | wc -l | tr -d ' ' || echo 0)
+    if [ "$NEW" -gt 0 ]; then
+      TOTAL=$((TOTAL + NEW))
+      ERR_5XX=$((ERR_5XX + $(awk -v cutoff="$CUTOFF" '$0 >= cutoff && /"status":5[0-9][0-9]/ {print}' "$LOG" 2>/dev/null | wc -l | tr -d ' ' || echo 0)))
+    fi
+  fi
+done
+
+if [ "$TOTAL" -gt 100 ]; then
+  RATE=$(awk "BEGIN {printf \"%.1f\", $ERR_5XX*100/$TOTAL}")
+  if [ "$(echo "$RATE > 5" | bc 2>/dev/null || echo 0)" = "1" ]; then
+    echo "{\"health\":\"degraded\",\"signalKind\":\"caddy-errors\",\"evidence\":\"${RATE}% 5xx rate ($ERR_5XX/$TOTAL)\",\"metrics\":{\"caddy_5xx_rate\":$RATE,\"caddy_requests\":$TOTAL,\"caddy_5xx\":$ERR_5XX}}"
+    exit 0
+  fi
+  echo "{\"health\":\"healthy\",\"metrics\":{\"caddy_5xx_rate\":$RATE,\"caddy_requests\":$TOTAL,\"caddy_5xx\":$ERR_5XX}}"
+else
+  echo '{"health":"healthy"}'
+fi