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>

scripts/install.py

@@ -11,7 +11,7 @@ This script handles first-time installation of the Wizamart platform:
 6. Provides a configuration status report
 
 Usage:
-    make install
+    make platform-install
     # or directly:
     python scripts/install.py
 
@@ -572,7 +572,7 @@ def main():
 
     if warning_count > 0 or missing_count > 0:
         print("  2. Update .env with production values")
-        print("  3. Run 'make install' again to verify")
+        print("  3. Run 'make platform-install' again to verify")
     else:
         print("  2. Start the application: make dev")