Change queue_schedules FK to ON DELETE SET NULL

The cleanup-queue-jobs job was failing because it tried to delete
completed/failed jobs still referenced by `queue_schedules.last_scheduled_job_id`.
The FK defaulted to RESTRICT, blocking the delete.

Change it to `ON DELETE SET NULL` so cleanup can proceed, matching the
existing semantics (NULL = "never scheduled or cleaned up").

Fixes #5545

crates/storage-pg/migrations/20260324000001_queue_schedules_last_job_set_null.sql

@@ -0,0 +1,14 @@
+-- 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.
+
+-- Change the FK constraint on last_scheduled_job_id to SET NULL on delete
+-- This allows us to clean up old completed/failed queue jobs without violating
+-- the FK constraint from queue_schedules (fixes #5545)
+ALTER TABLE queue_schedules
+  DROP CONSTRAINT queue_schedules_last_scheduled_job_id_fkey,
+  ADD CONSTRAINT queue_schedules_last_scheduled_job_id_fkey
+    FOREIGN KEY (last_scheduled_job_id)
+    REFERENCES queue_jobs (queue_job_id)
+    ON DELETE SET NULL;