Remove hardcoded dependency between artifacts and projects

[JAORMX]

Summary

This makes repos nullable in artifacts. As of today, this is not really
a case that actually happens... so this should effectively be a no-op.
However, the idea is that soon we'll start getting artifacts that that
come from other providers that are not the same as the repository they
are linked to.

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 48.323% (-0.05%) from 48.373%
when pulling da8ab74 on artifacts-projects
into af5fcf3 on main.

[dmjb]

Approving. It's unfortunate that we need to add the provider name to the artifacts table, but it's a temporary approach so I think it's fine for now.

[jhrozek]

this works for me!

[jhrozek]

this works even better for me!