Fix thread_subscriptions stream sequence

Works around https://github.com/element-hq/synapse/issues/18712

synapse/storage/schema/main/delta/92/08_thread_subscriptions_seq_fixup.sql.postgres

@@ -0,0 +1,19 @@
+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2025 New Vector, Ltd
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- See the GNU Affero General Public License for more details:
+-- <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+-- Work around https://github.com/element-hq/synapse/issues/18712 by advancing the
+-- stream sequence.
+-- This makes last_value of the sequence point to a position that will not get later
+-- returned by nextval.
+-- (For blank thread subscription streams, this means last_value = 2, nextval() = 3 after this line.)
+SELECT nextval('thread_subscriptions_sequence');