fix: ensure correct type when filtering events

[grnd-alt]

resolves #6858

caused because id in activity.subject_rich[1].board.id for card events is a string now: #6898
but object_id in board events is still a number, so need a type cast in the filtering.
open for discussion to just change it in the php code to make it uniform, but this pr also works.

[grnd-alt]

/backport to stable31

[backportbot]

The backport to stable31 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable31
git pull origin stable31

# Create the new backport branch
git checkout -b backport/7071/stable31

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 3b620154

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/7071/stable31

Error: Failed to cherry pick commits: error: no cherry-pick or revert in progress
fatal: cherry-pick failed

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.