fix: resolve Makefile duplicate install target and Celery deprecation warning

- Rename second 'install' target to 'platform-install' to avoid conflict
- Add broker_connection_retry_on_startup=True for Celery 6.0 compatibility
- Update install.py references to use 'make platform-install'

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/core/celery_config.py

@@ -43,6 +43,8 @@ celery_app.conf.update(
     # Timezone
     timezone="Europe/Luxembourg",
     enable_utc=True,
+    # Broker connection
+    broker_connection_retry_on_startup=True,  # Retry connection on startup (Celery 6.0+)
     # Task behavior
     task_track_started=True,
     task_time_limit=30 * 60,  # 30 minutes hard limit