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.

dashboard/src/pages/NewsFeed.tsx

@@ -8,7 +8,7 @@ interface Article {
   title: string;
   source: string;
   url: string;
-  published_at: string;
+  published_at: string | null;
   ticker: string;
   sentiment_score: number;
   confidence: number;
@@ -105,7 +105,9 @@ export default function NewsFeed() {
                     <span>{article.source}</span>
                     <span>|</span>
                     <span>
-                      {new Date(article.published_at).toLocaleString()}
+                      {article.published_at
+                        ? new Date(article.published_at).toLocaleString()
+                        : 'Date unknown'}
                     </span>
                     {article.ticker && (
                       <>

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,