fix(Item): fix ranking by invoicePaidAt instead of created_at

[brymut]

Description

Factor in invoicePaidAt instead of just created_at when ranking posts.
fixes #2596

Screenshots

To test, I recreated the post details from https://stacker.news/items/1246127 on a test post. And checked that the post appears as expected.

Checklist

Are your changes backward compatible? Please answer below:
Yes
For example, a change is not backward compatible if you removed a GraphQL field or dropped a database column.

On a scale of 1-10 how well and how have you QA'd this change and any features it might affect? Please answer below:
8

For frontend changes: Tested on mobile, light and dark mode? Please answer below:

Did you introduce any new environment variables? If so, call them out explicitly here:
N

Did you use AI for this? If so, how much did it assist you?
N

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Note

Use invoicePaidAt (fallback to created_at) for item time filtering and the recent query WHERE clause.

• Backend (GraphQL resolvers)
  • api/resolvers/item.js:
    • Update whenClause to use COALESCE("invoicePaidAt", created_at) for time range filtering.
    • In items resolver (sort: 'recent'), change WHERE to COALESCE("Item"."invoicePaidAt", "Item".created_at) <= $1 to align with ordering by paid time.

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