Add app_type field to App model (desktop/web/service)

[qzMalekuz]

Summary

The current App model has no way to distinguish between Desktop, Web,
and Service Apps. This is the foundational schema change needed to turn
the App Store into a unified multi-platform catalog.

This PR introduces an app_type field to the App model with three
supported values: desktop, web, and service.

Changes

• apps/models.py: Add APP_TYPE_* constants, APP_TYPE_CHOICES,
  and app_type = CharField(max_length=10, choices=..., default='desktop', db_index=True) to the App model
• dbmigration/add_app_type.py: Raw SQL migration script for existing
  deployments — defaults all existing rows to 'desktop' for full backward
  compatibility
• apps/tests.py: AppTypeTestCase covering default value, persistence
  for all three types, invalid value rejection via full_clean(), and
  queryset filtering

Backward Compatibility

All existing App rows default to 'desktop'. No existing functionality
is affected.

Context

This is the Week 1 deliverable from the GSoC 2026 proposal for the
multi-platform catalog project. Follow-up PRs will add ServiceAppMetadata,
admin filters, health monitoring, and the /backend/service_apps/ API
endpoint.

Closes #128

[qzMalekuz]

Hi @coleslaw481 @d2fong @jingjingbic,

This is the foundational schema change from my GSoC 2026 proposal (multi-platform catalog project).
The app_type field replaces the insufficient is_service boolean and supports three platform states: desktop(default), web, and service.
All existing rows default to desktop so nothing breaks.

Happy to adjust the field design, choices, or migration approach based on your feedback before I move on to Week 2 (ServiceAppMetadata + admin filters).

Thanks,
Zafar