fix: Umami externe sur stats.open.us.org — supprime docker-compose.umami.yml

- host corrigé : stats.open.us.org (instance partagée, non self-hosted)
- docker-compose.umami.yml supprimé (obsolète)
- .env.example simplifié (plus de vars self-hosted)
- README + CONTRIBUTING mis à jour

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

nuxt.config.ts

@@ -19,7 +19,7 @@ export default defineNuxtConfig({
   ],
 
   umami: {
-    host: 'https://stats.librodrome.org',
+    host: 'https://stats.open.us.org',
     id: '95ff616d-9ce1-47d9-bca2-f6ddc344a99a',
     autoTrack: true,
     ignoreLocalhost: true,