feat: enable Watchtower auto-updates for all application services

Add missing Watchtower labels to:
- net_umami: Analytics service
- dev_gitea_runner: CI/CD runner
- sexy_api: Directus CMS backend
- util_linkwarden_meilisearch: Search engine

All application services now have automatic updates enabled.
Critical infrastructure (postgres, redis, traefik) intentionally
excluded from auto-updates for stability.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

util/compose.yaml

@@ -127,6 +127,9 @@ services:
       MEILI_NO_ANALYTICS: ${UTIL_LINKS_MEILI_NO_ANALYTICS:-true}
     volumes:
       - linkwarden_meili_data:/meili_data
+    labels:
+      # Watchtower
+      - 'com.centurylinklabs.watchtower.enable=${WATCHTOWER_LABEL_ENABLE}'
 
   # Mattermost - Team collaboration
   mattermost: