Posts should "know" what site they are posted to

[official-sounding]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://ask.metastaging.net/345/commodi-rerum-nihil-laborum-et-quis#comments
2. Change URL to https://www.metastaging.net/345/commodi-rerum-nihil-laborum-et-quis#comments
3. Note that site changes to Metafilter, but body remains the same

Expected behavior
Probably a 301 Redirect to the URL from step 1

Desktop (please complete the following information):

• OS: Windows 11
• Browser Firefox
• Version 138

Additional context
Somewhat of a similar issue to #53