Fix essentially redundant per-platform columns in contributors table

> [!NOTE]
> Migrated from [augurlabs/augur#3363](https://github.com/augurlabs/augur/issues/3363)
> Originally opened by `@MoralCode` on 2025-10-31

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By my understanding the contributors table is updated in essentially two parts: half is updated by tasks that fetch data from github (core), and the other half is updated by data from git (facade).

when only half is updated, the other half of the data is null.

This is not a great database design, especially given that this table in particular is already very highly contested (#122).

I think this table should be split into two because each of the processes seems to only deal with a separate, only slightly overlapping, set of data anyway - a relationship that may be better served by a foreign key.