fix: news articles showing 1970 dates when published_at is null

Backend falls back to fetched_at when published_at is NULL in the database,
so the API always returns a meaningful date. Frontend also handles null
defensively to avoid new Date(null) producing Unix epoch 0.

services/api_gateway/routes/news.py

@@ -73,7 +73,7 @@ async def list_news(
                     "published_at": (
                         article.published_at.isoformat()
                         if article.published_at
-                        else None
+                        else article.fetched_at.isoformat()
                     ),
                     "fetched_at": article.fetched_at.isoformat(),
                     "ticker": sentiment.ticker,