feat: auto-trigger scheduled agents + fix stale nextRun display

- Add background scheduler in entrypoint: calls /api/cron/agents every
  5 minutes to trigger due agents automatically
- Recalculate stale nextRun (past dates) in getAgents() so the display
  always shows a future time
- Agent card shows "En attente de déclenchement" instead of "il y a X"
  when nextRun is in the past

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/locales/fr.json

@@ -1252,6 +1252,7 @@
     },
     "schedule": {
       "nextRun": "Prochaine exécution",
+      "pending": "En attente de déclenchement",
       "time": "Heure",
       "dayOfWeek": "Jour de la semaine",
       "dayOfMonth": "Jour du mois",