Fix dnsmasq reboot cron: use /etc/cron.d/ instead of crontab

Ansible cron module writes to root's crontab which requires cron
daemon running. Drop file in /etc/cron.d/ instead for reliability.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

playbook/pxe_server_setup.yml

@@ -620,11 +620,11 @@
         policy: deny
 
     - name: "Schedule dnsmasq restart 15s after reboot"
-      cron:
-        name: "Restart dnsmasq after reboot"
-        user: root
-        special_time: "reboot"
-        job: "/bin/sleep 15 && /usr/bin/systemctl restart dnsmasq.service"
+      copy:
+        dest: /etc/cron.d/dnsmasq-restart
+        mode: '0644'
+        content: |
+          @reboot root /bin/sleep 15 && /usr/bin/systemctl restart dnsmasq.service
 
     # --- Web Management App (Flask) ---
     - name: "Create webapp directory"