fix(multimodal): support HTML image tags in document extraction and chat responses

- Replace plain image URLs with `<img src="..." data-url="...">` HTML tags in multimodal and document extractor services
- Propagate citations from workflow end events to client responses
- Update system prompts to instruct LLMs to render images using Markdown `![alt](url)` with strict UUID-preserving URL copying

api/app/core/workflow/nodes/document_extractor/node.py

@@ -182,7 +182,7 @@ class DocExtractorNode(BaseNode):
                                     mime_type=f"image/{ext}",
                                     is_file=True,
                                 ).model_dump())
-                                text = text + f"\n{placeholder}: {url}"
+                                text = text + f"\n{placeholder}: <img src=\"{url}\" data-url=\"{url}\">"
                             except Exception as e:
                                 logger.error(f"Node {self.node_id}: failed to save image {placeholder}: {e}")