events-backend: add index for event topics

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

plugins/events-backend/migrations/20240523100528_init.js

@@ -48,6 +48,8 @@ exports.up = async function up(knex) {
         .comment(
           'The IDs of the subscribers that have already consumed this event',
         );
+
+      table.index('topic', 'event_bus_events_topic_idx');
     });
 
     await knex.schema.createTable('event_bus_subscriptions', table => {