- StockTwits and Reddit were fetched with no date, so a historical run showed
today's chatter as if it were from the as-of date
- pass the analysis window to both fetchers, filter to it, and emit a clear
placeholder when nothing qualifies
- centralize the UTC half-open window rule in dataflows/date_window so news,
StockTwits, and Reddit share one look-ahead-safe filter #1220
