The function returns Optional[str] today, with None conflating five
distinct outcomes (missing args / push disabled / unserialisable /
Redis down / XADD failed). Every current call site is fire-and-
forget and ignores the return, so the right move is to document the
five cases rather than promote to an enum return — keeps the API
small while making the diagnostic surface (logs) obvious. If a
future caller needs to react differently per reason, promote then.
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