feat(*): make message processing sequential

backend/src/bot/user_lock.py

@@ -0,0 +1,11 @@
+import asyncio
+
+_locks: dict[str, asyncio.Lock] = {}
+
+
+def get_user_lock(convex_user_id: str) -> asyncio.Lock:
+    lock = _locks.get(convex_user_id)
+    if lock is None:
+        lock = asyncio.Lock()
+        _locks[convex_user_id] = lock
+    return lock