Add PromptFeedback API for per-message thumbs ratings (#67) (#70)

## Summary
- Closes [#67](#67) — new `PromptFeedback` model (unique on `(prompt, user)`) with `rating` (`up`|`down`), optional `reason` / `comment`, and timestamps via `TimeInfoBase`.
- `POST /api/prompt_feedback` upserts `{ prompt_id, rating, reason?, comment? }`; `DELETE /api/prompt_feedback?prompt_id=` clears the caller's vote.
- `GET conversation_details` now nests the caller's `feedback: { rating, reason, comment }` (or `null`) on each prompt so [chat_web_app#101](ai_ml_operations/chat_web_app#101) can rehydrate thumbs UI.
- Auth required; users can only rate assistant prompts in their own non-deleted conversations. Distinct from app-wide `POST /feedbacks/`.
- Joinable to `PromptMetric` via `prompt_id` for per-model accuracy slices.

## Test plan
- [ ] `uv run python manage.py test chat_backend.tests.test_views_prompt_feedback`
- [ ] Upsert thumbs up, then down with reason/comment — one row updated
- [ ] DELETE clears vote; second DELETE → 404
- [ ] Rating another user's prompt → 404; rating a user message → 400
- [ ] Reload conversation details → assistant prompts show caller feedback only
- [ ] Companion FE [chat_web_app#101](ai_ml_operations/chat_web_app#101) thumbs + reason popover against this APIReviewed-on: #70

llm_be/chat_backend/serializers.py

@@ -8,6 +8,7 @@ from .models import (
     Company,
     Conversation,
     Prompt,
+    PromptFeedback,
     PromptMetric,
     Feedback,
     FEEDBACK_CATEGORIES,
@@ -210,9 +211,52 @@ class ConversationSerializer(serializers.ModelSerializer):
         return tout
 
 
+class PromptFeedbackSerializer(serializers.ModelSerializer):
+    prompt_id = serializers.IntegerField(source="prompt.id", read_only=True)
+
+    class Meta:
+        model = PromptFeedback
+        fields = (
+            "id",
+            "prompt_id",
+            "rating",
+            "reason",
+            "comment",
+            "created",
+            "last_modified",
+        )
+        read_only_fields = ("id", "prompt_id", "created", "last_modified")
+
+
+class PromptFeedbackUpsertSerializer(serializers.Serializer):
+    prompt_id = serializers.IntegerField()
+    rating = serializers.ChoiceField(choices=PromptFeedback.Rating.choices)
+    reason = serializers.ChoiceField(
+        choices=PromptFeedback.Reason.choices,
+        required=False,
+        allow_null=True,
+        allow_blank=True,
+    )
+    comment = serializers.CharField(
+        required=False, allow_null=True, allow_blank=True, max_length=1024
+    )
+
+    def validate_reason(self, value):
+        if value == "":
+            return None
+        return value
+
+    def validate_comment(self, value):
+        if value is None:
+            return None
+        stripped = str(value).strip()
+        return stripped or None
+
+
 class PromptSerializer(serializers.ModelSerializer):
     tokens_in = serializers.SerializerMethodField()
     tokens_out = serializers.SerializerMethodField()
+    feedback = serializers.SerializerMethodField()
 
     class Meta:
         model = Prompt
@@ -224,8 +268,9 @@ class PromptSerializer(serializers.ModelSerializer):
             "tokens_in",
             "tokens_out",
             "citations",
+            "feedback",
         )
-        read_only_fields = ("citations",)
+        read_only_fields = ("citations", "feedback")
 
     def _token_pair(self, obj):
         cache = self.context.setdefault("_prompt_token_cache", {})
@@ -241,6 +286,34 @@ class PromptSerializer(serializers.ModelSerializer):
         _, tout = self._token_pair(obj)
         return tout
 
+    def get_feedback(self, obj):
+        """Current caller's rating for this prompt, if any."""
+        request = self.context.get("request")
+        if request is None or not getattr(request, "user", None):
+            return None
+        user = request.user
+        if not user.is_authenticated:
+            return None
+
+        by_prompt = self.context.get("_prompt_feedback_by_id")
+        if by_prompt is None:
+            prompt_ids = self.context.get("_prompt_ids_for_feedback")
+            qs = PromptFeedback.objects.filter(user=user).only(
+                "prompt_id", "rating", "reason", "comment"
+            )
+            if prompt_ids is not None:
+                qs = qs.filter(prompt_id__in=prompt_ids)
+            by_prompt = {
+                row.prompt_id: {
+                    "rating": row.rating,
+                    "reason": row.reason,
+                    "comment": row.comment,
+                }
+                for row in qs
+            }
+            self.context["_prompt_feedback_by_id"] = by_prompt
+        return by_prompt.get(obj.id)
+
     def validate_message(self, value: str) -> str:
         if value is None or not str(value).strip():
             raise serializers.ValidationError("Message text cannot be empty.")