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/time_check.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+# time_check.sh — NTP synchronization status and clock drift.
+set -euo pipefail
+
+DRIFT_S=0
+SYNCED=true
+
+if command -v chronyc >/dev/null 2>&1; then
+  TRACKING=$(chronyc tracking 2>/dev/null || true)
+  DRIFT_NS=$(echo "$TRACKING" | awk '/System time/ {print $4}' | sed 's/-//' || echo "0")
+  DRIFT_S=$(awk "BEGIN {printf \"%.6f\", $DRIFT_NS/1e9}")
+  # chronyc returns a very small value when synced (nanoseconds)
+elif command -v timedatectl >/dev/null 2>&1; then
+  STATUS=$(timedatectl show 2>/dev/null || true)
+  if echo "$STATUS" | grep -q "NTPSynchronized=no"; then
+    SYNCED=false
+  fi
+fi
+
+if ! $SYNCED; then
+  echo '{"health":"degraded","signalKind":"time-drift","evidence":"NTP not synchronized"}'
+elif [ "$(echo "$DRIFT_S > 1" | bc 2>/dev/null || echo 0)" = "1" ]; then
+  echo "{\"health\":\"degraded\",\"signalKind\":\"time-drift\",\"evidence\":\"clock drift ${DRIFT_S}s exceeds 1s threshold\",\"metrics\":{\"clock_drift_s\":$DRIFT_S}}"
+else
+  echo "{\"health\":\"healthy\",\"metrics\":{\"clock_drift_s\":$DRIFT_S}}"
+fi