POPs can have dangling pointers

Consider the following scenario:

1. I make a new PEP called nleroy917/fine-tuning:default
2. I attach this to a POP called nleroy917/geniml:default
3. I delete nleroy917/fine-tuning:default

The POP now points to a PEP that doesn't exist. Rust would call this a dangling reference. The POP is just a sample table that points to a list of PEPs. However, there are no safeguards in place if that PEP no longer exists.

@khoroshevskyi and I have talked about this, but there's no clear solution, so it should probably be addressed in the future to prevent weird behavior. Some options:

Option 1 (currently):

Do nothing

Option 2:

Check if that PEP belongs to a POP, and prevent users from deleting it.

Option 3:

Check if that PEP belongs to a POP, and warn users before deleting.

Option 4:

Implement some sort of system that propagates the fact that a PEP has been removed from all POPs that contain that PEP and update their sample tables accordingly.

Option 4 is the most ideal. Options 2 and 3 are a good start, but the query to implement them takes too long. Option 1 is not ideal.

No files, tests, or entry points are named. Start by tracing how PEP deletion and POP sample-table relationships are represented, then compare the proposed deletion safeguards and propagation behavior. Done means deleting a PEP no longer leaves any POP pointing to a nonexistent PEP, with coverage for the chosen behavior.