fix: configure Watchtower to only monitor agent container

Add WATCHTOWER_SCOPE=towerops environment variable to generated
docker-compose templates. This prevents Watchtower from monitoring
unrelated containers on the same Docker host (e.g., Dokku apps).

Without scope filtering, Watchtower would attempt to monitor all
containers with watchtower.enable=true label, causing warnings
for missing images and unnecessary monitoring overhead.

lib/towerops_web/live/agent_live/index.html.heex

@@ -478,6 +478,7 @@
       # Check for updates every 12 hours (43200 seconds)
       - WATCHTOWER_POLL_INTERVAL=43200
       - WATCHTOWER_LABEL_ENABLE=true
+      - WATCHTOWER_SCOPE=towerops
       - WATCHTOWER_CLEANUP=true
       - WATCHTOWER_LOG_LEVEL=info
       # Use latest Docker API version

lib/towerops_web/live/help_live/index.ex

@@ -990,6 +990,7 @@ defmodule ToweropsWeb.HelpLive.Index do
     environment:
       - WATCHTOWER_POLL_INTERVAL=43200
       - WATCHTOWER_LABEL_ENABLE=true
+      - WATCHTOWER_SCOPE=towerops
       - WATCHTOWER_CLEANUP=true
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock]) %></code></pre>