Add _fetch_many() helper using ThreadPoolExecutor to query instances
in parallel. Refactors PeerTube, Mastodon, Lemmy, and SearXNG from
sequential to concurrent fetches. Also adds retries parameter to
derp.http.urlopen; multi-instance backends use retries=1 since
instance redundancy already provides resilience.
Worst-case wall time per backend drops from N*timeout to 1*timeout.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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