feat(threads): add basic message thread API and web thread panel

app/messages/service.py

@@ -177,6 +177,20 @@ async def search_messages(
     )
 
 
+async def get_message_thread(
+    db: AsyncSession,
+    *,
+    message_id: int,
+    user_id: int,
+    limit: int = 100,
+) -> list[Message]:
+    root = await repository.get_message_by_id(db, message_id)
+    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)
+
+
 async def update_message(
     db: AsyncSession,
     *,