Fix AttributeError in maybe_archive_idle_post when post is missing

[oliveman-au]

When get_or_fetch_channel raises discord.HTTPException, the except block references post and post.id — but post was never assigned since the exception occurred before the assignment completed. This causes an AttributeError: 'int' object has no attribute 'id', meaning the error handler itself crashes instead of recovering gracefully.

Fixed by replacing post and post.id with post_id in the log line, which is the integer that was passed into the function.

[L3viathan]

LGTM