Add exclude_attending to event search query

[nabramow]

I didn't realize when I did the other that this is a separate query (since this one supports more searching params). Adding it here too.

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• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
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[nabramow]

The other one had include_attending which was a bit different but I think it still doesn't make sense to allow both at the same time?

[CouchersBot]

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[nabramow]

an empty OR means "no inclusion filter" (return everything), not "return nothing"?

[github-actions]

📝 Release Notes

This PR should be included in the release notes.

Suggested release note:

Added an option to exclude events you're already attending or organizing from event search results

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: include
Reasoning: This adds a new event search filter that can noticeably improve how users browse events, especially by hiding events they are already attending or organizing. It is a user-facing search capability rather than an internal-only change.