Add index on (user_id, shown) in notification table to optimize markNotificationsShown queries and prevent deadlocks. Implement transaction handling in markNotificationsShown method for atomic updates.

backend/migrations/20260118000000-add-notification-user-shown-index.cjs

@@ -0,0 +1,30 @@
+"use strict";
+
+module.exports = {
+  async up(queryInterface, Sequelize) {
+    // Create index on (user_id, shown) to optimize markNotificationsShown queries
+    // This prevents deadlocks by allowing fast lookups and reducing lock contention
+    await queryInterface.sequelize.query(`
+      DO $$
+      BEGIN
+        IF NOT EXISTS (
+          SELECT 1 FROM pg_class c 
+          JOIN pg_namespace n ON n.oid = c.relnamespace
+          WHERE c.relkind = 'i' 
+            AND c.relname = 'idx_notification_user_id_shown' 
+            AND n.nspname = 'falukant_log'
+        ) THEN
+          CREATE INDEX idx_notification_user_id_shown 
+          ON falukant_log.notification (user_id, shown);
+        END IF;
+      END$$;
+    `);
+  },
+
+  async down(queryInterface, Sequelize) {
+    await queryInterface.sequelize.query(`
+      DROP INDEX IF EXISTS falukant_log.idx_notification_user_id_shown;
+    `);
+  }
+};
+