refactor: improve markdown sanitization by collapsing excessive newlines

backend/main.py

@@ -133,7 +133,8 @@ def _sanitize_converted_markdown(text: str) -> str:
     value = (text or "").replace("\r\n", "\n").replace("\r", "\n")
     value = IMAGE_MARKDOWN_RE.sub("", value)
     value = IMAGE_HTML_RE.sub("", value)
-    return value
+    value = re.sub(r"\n{3,}", "\n\n", value)
+    return value.strip()
 
 
 def get_client_ip(request: Request) -> str: