Add indexes for quarantined media lookups

[turt2live]

This is intended to overlap with #19308

Lack of index was introduced in #19268

We require an index on these lookups because early EXPLAIN results on matrix.org indicate that it'd likely destroy performance if we try to get the first page of results. The cost is far too high (>50k minimum).

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.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
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• Code style is correct (run the linters)