feat(_update): replace watchtower with custom nightly update script

Removes the watchtower container in favour of a host-side script that
runs daily at 2:00 AM via systemd timer.  Mirrors the _backup pattern:
auto-discovers stacks, pulls images, recreates changed containers,
prunes dangling images, and notifies via n8n → Telegram.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

_update/stacks-update.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Daily Docker stacks image update at 2:00 AM
+
+[Timer]
+OnCalendar=*-*-* 02:00:00 Europe/Amsterdam
+Persistent=true
+
+[Install]
+WantedBy=timers.target