Enforce one product per purchasable object

[cp-at-mit]

What are the relevant tickets?

https://github.com/mitodl/hq/issues/11729

Description (What does it do?)

Updates Product uniqueness to always enforce a single product per (object_id, content_type) pair, instead of allowing duplicates when one is inactive. Adds a migration to replace the old conditional constraint and updates the model test to cover both active and inactive duplicate creation attempts.

How can this be tested?

Create a product, then attempt to create a second active product for the same object. This should not work even if the first product is deactivated.

[github-actions]

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