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/memory_check.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# memory_check.sh — RAM usage percentage.
+set -euo pipefail
+
+TOTAL=1
+AVAIL=1
+
+if [ -r /proc/meminfo ]; then
+  TOTAL=$(awk '/^MemTotal:/ {printf "%d", $2}' /proc/meminfo)
+  AVAIL=$(awk '/^MemAvailable:/ {printf "%d", $2}' /proc/meminfo)
+elif [ "$(uname)" = "Darwin" ]; then
+  MEM=$(vm_stat 2>/dev/null | awk '
+    /page size/ {ps=$8}
+    /Pages free/   {free+=$NF}
+    /Pages active/ {active+=$NF}
+    /Pages wired/  {wired+=$NF}
+    END {printf "%.2f %.2f", ps*(free+active+wired)/1048576, ps*wired/1048576}')
+  TOTAL=$(echo "$MEM" | awk '{printf "%.0f", $1}')
+  AVAIL=$(echo "$MEM" | awk '{printf "%.0f", $1 - $2}')
+fi
+
+if [ "$TOTAL" -eq 0 ]; then TOTAL=1; fi
+if [ "$AVAIL" -lt 0 ]; then AVAIL=0; fi
+
+USED_PCT=$(awk "BEGIN {printf \"%.1f\", (1 - $AVAIL/$TOTAL)*100}")
+
+echo "{\"health\":\"healthy\",\"metrics\":{\"mem_pct\":$USED_PCT}}"