Add trigger and backfill for upstream OAuth user session tracking

Introduce a new trigger and a backfill migration to populate the
`user_session_id` column in `upstream_oauth_authorization_sessions`
based on `user_session_authentications`. This ensures historical data is
consistent and aids in backward compatibility.

crates/storage-pg/migrations/20260121121140_upstream_oauth_track_user_session_trigger.sql

@@ -0,0 +1,27 @@
+-- Copyright 2026 Element Creations Ltd.
+--
+-- SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+-- Please see LICENSE files in the repository root for full details.
+
+-- Adds a trigger which will backfill the user_session_id column when inserting
+-- a new user_session_authentications row. This is to help supporting rolling
+-- back to previous releases and should be dropped in a future version.
+CREATE OR REPLACE FUNCTION upstream_oauth_authorization_sessions_insert_trigger()
+RETURNS TRIGGER AS $$
+BEGIN
+    IF NEW.upstream_oauth_authorization_session_id IS NOT NULL THEN
+        UPDATE upstream_oauth_authorization_sessions
+        SET user_session_id = NEW.user_session_id
+        WHERE upstream_oauth_authorization_sessions.upstream_oauth_authorization_session_id
+               = NEW.upstream_oauth_authorization_session_id
+          AND upstream_oauth_authorization_sessions.user_session_id IS NULL;
+    END IF;
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+-- Create the trigger
+CREATE TRIGGER upstream_oauth_authorization_sessions_insert_trigger
+  AFTER INSERT ON user_session_authentications
+  FOR EACH ROW
+  EXECUTE FUNCTION upstream_oauth_authorization_sessions_insert_trigger();

crates/storage-pg/migrations/20260121121150_upstream_oauth_track_user_session_backfill.sql

@@ -0,0 +1,13 @@
+-- Copyright 2026 Element Creations Ltd.
+--
+-- SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+-- Please see LICENSE files in the repository root for full details.
+
+-- Backfill the upstream_oauth_authorization_sessions.user_session_id column
+-- based on session authentications
+UPDATE upstream_oauth_authorization_sessions
+SET user_session_id = user_session_authentications.user_session_id
+FROM user_session_authentications
+WHERE upstream_oauth_authorization_sessions.user_session_id IS NULL
+  AND upstream_oauth_authorization_sessions.upstream_oauth_authorization_session_id
+             = user_session_authentications.upstream_oauth_authorization_session_id;