Events: Rename EventOfflineInformation to EventLocation

[tristanlabelle]

Follow-up to #9133 . Since we don't have online events anymore, we can rename EventOfflineInformation to the more descriptive EventLocation. This is safe as names are not part of the serialized protobuf format.

Testing

Ran linting+mypy

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

Web frontend checklist

• There are no console warnings when running the app
• Added tests where relevant
• Clicked around my changes running locally and it works
• Checked Desktop, Mobile and Tablet screen sizes

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

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Backend

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Other

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[aapeliv]

lgtm on backend side as it's just a rename

[github-actions]

📝 Release Notes

This PR does not need to be included in release notes.

Reason: This PR is an internal rename of event location fields and protobuf messages after online events were already removed in an earlier PR. It does not introduce a new user-facing feature or noticeable behavior change, and the backcompat notes indicate it is safe internal cleanup rather than a significant technical improvement for users.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This PR is an internal rename of event location fields and protobuf messages after online events were already removed in an earlier PR. It does not introduce a new user-facing feature or noticeable behavior change, and the backcompat notes indicate it is safe internal cleanup rather than a significant technical improvement for users.