Add alt_text field to image content for Gradio validation

Gradio requires alt_text to be a string when present, so explicitly set it to "uploaded image" when creating image content.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

app.py

@@ -98,7 +98,7 @@ def user(user_message, image_data, history: list):
             "role": "user",
             "content": [
                 {"type": "text", "text": stripped_message},
-                {"type": "image", "path": tmp_path, "_pil_image": image_obj}
+                {"type": "image", "path": tmp_path, "alt_text": "uploaded image", "_pil_image": image_obj}
             ]
         })
         has_content = True
@@ -108,7 +108,7 @@ def user(user_message, image_data, history: list):
     elif tmp_path is not None:
         updated_history.append({
             "role": "user",
-            "content": [{"type": "image", "path": tmp_path, "_pil_image": image_obj}]
+            "content": [{"type": "image", "path": tmp_path, "alt_text": "uploaded image", "_pil_image": image_obj}]
         })
         has_content = True