Read attached chat images with a vision model (#82)

## Summary
- Image attachments (PNG, JPEG, WebP, GIF) skip the document parser and go to a vision-capable Ollama model (`OLLAMA_MODEL_VISION`, default `gemma4:latest`).
- The image is stored on the prompt. `PromptSerializer` returns `file_type` and base64 `image` so history can redraw it.
- Unsupported image types and files over 8 MB return a websocket error and are not saved.
- A later text turn does not reuse a previous image as a spreadsheet.

Pairs with `chat_web_app` `feat/128-vision-image-prompt` ([#128](ai_ml_operations/chat_web_app#128)).

## Test plan
- [ ] `manage.py test chat_backend.tests.test_vision chat_backend.tests.test_consumers.GraphNodeTestCase`
- [ ] Send a prompt plus PNG over `ws/chat_again/` and confirm the vision model is called.
- [ ] `GET conversation_details` includes `image` and `file_type` for that turn.
- [ ] CSV upload still follows the document path.

Reviewed-on: #82

llm_be/chat_backend/serializers.py

@@ -257,6 +257,7 @@ class PromptSerializer(serializers.ModelSerializer):
     tokens_in = serializers.SerializerMethodField()
     tokens_out = serializers.SerializerMethodField()
     feedback = serializers.SerializerMethodField()
+    image = serializers.SerializerMethodField()
 
     class Meta:
         model = Prompt
@@ -269,8 +270,10 @@ class PromptSerializer(serializers.ModelSerializer):
             "tokens_out",
             "citations",
             "feedback",
+            "file_type",
+            "image",
         )
-        read_only_fields = ("citations", "feedback")
+        read_only_fields = ("citations", "feedback", "file_type", "image")
 
     def _token_pair(self, obj):
         cache = self.context.setdefault("_prompt_token_cache", {})
@@ -314,6 +317,11 @@ class PromptSerializer(serializers.ModelSerializer):
             self.context["_prompt_feedback_by_id"] = by_prompt
         return by_prompt.get(obj.id)
 
+    def get_image(self, obj):
+        from chat_backend.services.vision import image_base64
+
+        return image_base64(obj)
+
     def validate_message(self, value: str) -> str:
         if value is None or not str(value).strip():
             raise serializers.ValidationError("Message text cannot be empty.")