[WIP] Add searchusername method to Postgres player repository

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

Create a method on the Postgres player repository called searchusername. This should take a search string and a top int and return a slice of up to top unique uuids that have been registered as having names matching the search term. The implementation should use trigram similarity to sort the matches with a secondary sort on the last time the user was recorded with that username.
Add table tests for this method that set up a schema with a given list of db entries and then queries with a search term and top value and asserts the result.

Create an app method that simply wraps this method, and create very simple tests for it. Refer to e.g. the app method for get tags for inspiration.

Then create a get endpoint that serves this app method. Add simple tests for the endpoint. Refer to the get tags endpoint for inspiration.

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