uvicorn's default --timeout-keep-alive is 5s, shorter than Caddy's pooled
keepalive idle window, so the proxy reused connections uvicorn had just
closed -> broken pipe / connection reset -> 502. The player's progress/state
POSTs fire every 5s (resonant with the 5s default) and, being non-idempotent,
could not be auto-retried by the proxy, so those 502s reached the user.
Set --timeout-keep-alive 75 so the upstream outlives the proxy's reuse.
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