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/journal_check.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+# journal_check.sh — count journal errors in the last check interval.
+set -euo pipefail
+
+ERRORS=0
+
+if command -v journalctl >/dev/null 2>&1; then
+  ERRORS=$(journalctl -p err --since "-5min" --no-pager 2>/dev/null | wc -l | tr -d ' ' || echo 0)
+fi
+
+if [ "$ERRORS" -gt 0 ]; then
+  echo "{\"health\":\"degraded\",\"signalKind\":\"journal-errors\",\"evidence\":\"$ERRORS error entries in last 5min\",\"metrics\":{\"journal_errors\":$ERRORS}}"
+else
+  echo '{"health":"healthy","metrics":{"journal_errors":0}}'
+fi