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.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=Docker stacks image update
+After=network-online.target docker.service
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+User=root
+ExecStart=/root/stacks/_update/update.sh
+Environment=HOME=/root