feat(threads): support nested replies in thread API and panel

app/messages/service.py

@@ -188,7 +188,29 @@ async def get_message_thread(
     if not root:
         raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Message not found")
     await ensure_chat_membership(db, chat_id=root.chat_id, user_id=user_id)
-    return await repository.list_message_thread(db, root_message_id=message_id, user_id=user_id, limit=limit)
+    safe_limit = max(1, min(limit, 500))
+    collected: dict[int, Message] = {root.id: root}
+    frontier: list[int] = [root.id]
+    remaining = safe_limit - 1
+    while frontier and remaining > 0:
+        batch = await repository.list_messages_by_parent_ids(
+            db,
+            chat_id=root.chat_id,
+            user_id=user_id,
+            parent_ids=frontier,
+            limit=min(remaining, 200),
+        )
+        next_frontier: list[int] = []
+        for message in batch:
+            if message.id in collected:
+                continue
+            collected[message.id] = message
+            next_frontier.append(message.id)
+            remaining -= 1
+            if remaining <= 0:
+                break
+        frontier = next_frontier
+    return sorted(collected.values(), key=lambda item: item.id)
 
 
 async def update_message(