feat: start resume sessions in new flow

backend/app/postgres_database.py

@@ -46,9 +46,18 @@ class PostgresDatabase:
             ))
             self.insert_turn(connection, session_id=session_id, **initial_turn)
 
-    def fetch_session(self, connection: Connection, session_id: str) -> dict[str, Any] | None:
+    def fetch_session(
+        self,
+        connection: Connection,
+        session_id: str,
+        *,
+        for_update: bool = False,
+    ) -> dict[str, Any] | None:
         sessions = self.tables["sessions"]
-        row = connection.execute(select(sessions).where(sessions.c.id == session_id)).mappings().first()
+        statement = select(sessions).where(sessions.c.id == session_id)
+        if for_update:
+            statement = statement.with_for_update()
+        row = connection.execute(statement).mappings().first()
         if row is None:
             return None
         result = dict(row)