Added async lock to database transactions to prevent concurrent transaction conflicts.

crabstero/database.py

@@ -14,6 +14,7 @@
 
 """Async SQLite database access for Crabstero's persistent storage."""
 
+import asyncio
 from contextlib import asynccontextmanager
 from typing import TYPE_CHECKING, NamedTuple, Self
 
@@ -109,13 +110,16 @@ class Database:
         :param connection: An open aiosqlite connection.
         """
         self._connection = connection
+        self._tx_lock = asyncio.Lock()
 
     @asynccontextmanager
     async def _transaction(self) -> AsyncIterator[None]:
         """Begin an immediate write transaction.
 
-        Commits on success, rolls back on error.
+        Acquires an async lock so that only one transaction runs at a
+        time, then commits on success or rolls back on error.
         """
+        async with self._tx_lock:
             await self._connection.execute("BEGIN IMMEDIATE")
             try:
                 yield