fix: add watchtower label to runner service so auto-updates work

Without com.centurylinklabs.watchtower.enable=true the homelab watchtower
(running with --label-enable) silently skipped the runner container,
leaving it stuck on v2.5.60 while fixes accumulated on newer tags.

homelab/runner/docker-compose.yml

@@ -38,6 +38,8 @@ services:
     image: kalekber/libnovel-runner:latest
     restart: unless-stopped
     stop_grace_period: 135s
+    labels:
+      - "com.centurylinklabs.watchtower.enable=true"
     depends_on:
       - libretranslate
     # Pin prod subdomains to the prod server IP to bypass Cloudflare's 100s