Backchannel logout behavior settings on upstream providers

crates/storage-pg/migrations/20250630120643_upstream_oauth_on_backchannel_logout.sql

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+-- Copyright 2025 New Vector Ltd.
+--
+-- SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+-- Please see LICENSE in the repository root for full details.
+
+-- This defines the behavior when receiving a backchannel logout notification
+ALTER TABLE "upstream_oauth_providers"
+  ADD COLUMN "on_backchannel_logout" TEXT
+    NOT NULL
+    DEFAULT 'do_nothing';