fix: flush article before creating sentiment FK references

ArticleSentiment referenced db_article.id before the Article was flushed,
causing a NotNullViolationError on the article_id column. Adding an explicit
flush after session.add(db_article) ensures the UUID is populated before
creating the foreign key reference.

services/sentiment_analyzer/main.py

@@ -125,6 +125,7 @@ async def process_article(
                     content_hash=article.content_hash,
                 )
                 session.add(db_article)
+                await session.flush()
 
                 for ticker in tickers:
                     sentiment = ArticleSentiment(