Add composite index to discussion_topics table

closes VICE-5723
flag=none
Fixes slow queries when filtering by context and assignment
see DiscussionTopicsController#index:335

Test Plan:
- Jenkins passes
- Verify migration runs successfully
- index works

Change-Id: I4a8cb264d6449e875ef430fc2910d812be4bcb6e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/401014
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Migration-Review: Isaac Moore <isaac.moore@instructure.com>
Reviewed-by: Norbert Zsolt Bakó <norbert.bako@instructure.com>
QA-Review: Theo Sorriaux <theo.sorriaux@instructure.com>
Product-Review: Theo Sorriaux <theo.sorriaux@instructure.com>

db/migrate/20260213140311_add_context_id_id_index_to_discussion_topics.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+#
+# Copyright (C) 2026 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas 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, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+
+class AddContextIdIdIndexToDiscussionTopics < ActiveRecord::Migration[8.0]
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def change
+    add_index :discussion_topics,
+              [:context_id, :id],
+              name: "index_discussion_topics_on_context_id_and_id",
+              algorithm: :concurrently,
+              if_not_exists: true
+  end
+end