handler: use separate collection for comments instead of embedding

internal/handler/handler.go

···
 		ThreadID   string `json:"threadId"`
 		QuotedText string `json:"quotedText"`
 		Text       string `json:"text"`
+		ReplyTo    string `json:"replyTo,omitempty"` // parent comment URI for threading
 	}
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		http.Error(w, "Invalid request", http.StatusBadRequest)
···
 		ownerDID = req.OwnerDID
 	}
 
+	// Verify document exists (but don't fetch full content)
 	ownerClient, err := h.xrpcClient(ownerUserID)
 	if err != nil {
 		http.Error(w, "Failed to connect to owner PDS", http.StatusInternalServerError)
 		return
 	}
-
-	value, _, err := ownerClient.GetRecord(ownerDID, collectionDocument, rKey)
+	_, _, err = ownerClient.GetRecord(ownerDID, model.CollectionDocument, rKey)
 	if err != nil {
 		log.Printf("CommentCreate: GetRecord: %v", err)
 		http.Error(w, "Document not found", http.StatusNotFound)
 		return
 	}
-	var doc model.Document
-	if err := json.Unmarshal(value, &doc); err != nil {
-		http.Error(w, "Failed to parse document", http.StatusInternalServerError)
-		return
-	}
 
 	authorHandle, _ := atproto.ResolveHandleFromDID(session.DID)
 
···
 		threadID = randomID()
 	}
 
-	comment := model.EmbeddedComment{
-		ID:           randomID(),
+	// Create standalone comment record
+	comment := model.CommentRecord{
 		ThreadID:     threadID,
+		DocRKey:      rKey,
+		DocOwnerDID:  ownerDID,
 		QuotedText:   req.QuotedText,
 		Text:         req.Text,
 		Author:       session.DID,
 		AuthorHandle: authorHandle,
 		CreatedAt:    time.Now().UTC().Format(time.RFC3339),
+		ReplyTo:      req.ReplyTo,
+		Resolved:     false,
 	}
-	doc.Comments = append(doc.Comments, comment)
 
-	if _, _, err := ownerClient.PutDocument(rKey, &doc); err != nil {
-		log.Printf("CommentCreate: PutDocument: %v", err)
-		http.Error(w, "Failed to save comment", http.StatusInternalServerError)
+	// Create as separate record in app.diffdown.comment collection
+	uri, _, err := ownerClient.CreateRecord(model.CollectionComment, comment)
+	if err != nil {
+		log.Printf("CommentCreate: CreateRecord: %v", err)
+		http.Error(w, "Failed to create comment", http.StatusInternalServerError)
 		return
 	}
 
-	h.jsonResponse(w, comment, http.StatusCreated)
+	// Return response with URI for potential reply linking
+	response := struct {
+		model.CommentRecord
+		URI string `json:"uri"`
+	}{CommentRecord: comment, URI: uri}
+
+	h.jsonResponse(w, response, http.StatusCreated)
 }
 
 func (h *Handler) CommentList(w http.ResponseWriter, r *http.Request) {