fix: add file upload validation

Add check for None file_obj at start of file reading to show user-friendly
error message instead of Python TypeError when no file is uploaded.

app.py

@@ -765,6 +765,10 @@ def summarize_streaming(
     
     # Read uploaded file
     try:
+        if file_obj is None:
+            yield ("", "Error: Please upload a transcript file first", "", metrics)
+            return
+
         path = file_obj.name if hasattr(file_obj, 'name') else file_obj
         # Get file metadata
         import os