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>

Makefile

@@ -123,7 +123,7 @@ init-prod:
 	@echo "✨ Platform is ready for production OR development"
 
 # First-time installation - Complete setup with configuration validation
-install:
+platform-install:
 	@echo "🚀 WIZAMART PLATFORM INSTALLATION"
 	@echo "=================================="
 	$(PYTHON) scripts/install.py