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

internal/httpapi/gen/api.gen.go

@@ -81,6 +81,7 @@ const (
 	CheckKindDisk       CheckKind = "disk"
 	CheckKindDrift      CheckKind = "drift"
 	CheckKindHttp       CheckKind = "http"
+	CheckKindPing       CheckKind = "ping"
 	CheckKindSshScript  CheckKind = "ssh-script"
 	CheckKindTcp        CheckKind = "tcp"
 )
@@ -91,6 +92,7 @@ const (
 	CheckCreateKindDisk       CheckCreateKind = "disk"
 	CheckCreateKindDrift      CheckCreateKind = "drift"
 	CheckCreateKindHttp       CheckCreateKind = "http"
+	CheckCreateKindPing       CheckCreateKind = "ping"
 	CheckCreateKindSshScript  CheckCreateKind = "ssh-script"
 	CheckCreateKindTcp        CheckCreateKind = "tcp"
 )