Add ServiceAppMetadata model and admin panel enhancements

[qzMalekuz]

What's changed

Builds on #138. Implements Week 2 of my GSoC proposal.

apps/models.py

• New ServiceAppMetadata model (one-to-one with App, service type only)
  • Fields: service_url, service_spec_version, registration_validated,
    health_status, last_health_check
  • clean() rejects attachment to non-service apps

apps/admin.py

• AppAdmin with app_type/active/featured filters and search
• ServiceAppMetadataInline shown only for service-type apps; health fields are read-only

dbmigration/add_service_app_metadata.py

• Raw SQL script to create the table (consistent with existing dbmigration scripts)

build/.../0002_add_app_type_and_service_metadata.py

• Django migration for the test runner

45 tests passing.

[qzMalekuz]

Hey @coleslaw481 ,

This is the Week 2 deliverable from my GSoC proposal, building directly on #138.

The ServiceAppMetadata model lays the groundwork for the next two weeks:

• Week 3 will add the validation pipeline (util/service_validation.py) that flips registration_validated to True
• Week 4 will add the check_service_health management command that updates health_status and last_health_check

Happy to adjust the model fields or the admin layout based on feedback before those pieces land.