fix package tab 500 when multiple domains mapped to an app

[sukuna28]

Summary

• Overrides get_primary_domain() on TenantModel to use .filter(is_primary=True).first() instead of inheriting django-tenants' .get() which raises MultipleObjectsReturned when multiple domains are marked primary.
• Fixes 500 error on the package detail page when multiple domains are mapped to a single application.

Fixes #288

Test plan

• Map multiple domains to a single application
• Navigate to the package tab in App Panel
• Confirm package list loads without 500 error
• Confirm package configuration URLs resolve correctly