Add an Admin API endpoint for listing quarantined media

[turt2live]

Note: this was written using the copy/paste development technique, so may not adhere to great Python standards.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
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• Code style is correct (run the linters)

[turt2live]

looks like test flakes on not-my changes

[anoadragon453]

The trial and Build release artifacts CI failures look related to your changes, I'm afraid (typically it's only the Complement or Sytest runs that will flake):

[FAIL]
Traceback (most recent call last):
  File "/tmp/tmp.mV4uD1eGTx/tests/rest/admin/test_media.py", line 833, in test_list_quarantined_media
    self.assertEqual(200, channel.code, msg=channel.json_body)
  File "/synapse/build/debian/matrix-synapse-py3/opt/venvs/matrix-synapse/lib/python3.13/site-packages/twisted/trial/_synctest.py", line 444, in assertEqual
    super().assertEqual(first, second, msg)
  File "/usr/lib/python3.13/unittest/case.py", line 907, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/usr/lib/python3.13/unittest/case.py", line 900, in _baseAssertEqual
    raise self.failureException(msg)
twisted.trial.unittest.FailTest: 200 != 401 : {'errcode': 'M_MISSING_TOKEN', 'error': 'Missing access token'}

tests.rest.admin.test_media.ListQuarantinedMediaTestCase.test_list_quarantined_media

and looks due to this line not having an access token specified.