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/services/falukantService.js

@@ -4382,11 +4382,26 @@ class FalukantService extends BaseService {
 
     async markNotificationsShown(hashedUserId) {
         const user = await getFalukantUserOrFail(hashedUserId);
-        const [count] = await Notification.update(
-            { shown: true },
-            { where: { userId: user.id, shown: false } }
-        );
-        return { updated: count };
+        
+        // Use transaction to prevent deadlocks and ensure atomicity
+        const transaction = await sequelize.transaction({
+            isolationLevel: Sequelize.Transaction.ISOLATION_LEVELS.READ_COMMITTED
+        });
+        
+        try {
+            const [count] = await Notification.update(
+                { shown: true },
+                { 
+                    where: { userId: user.id, shown: false },
+                    transaction
+                }
+            );
+            await transaction.commit();
+            return { updated: count };
+        } catch (error) {
+            await transaction.rollback();
+            throw error;
+        }
     }
 
     async getPoliticalOfficeHolders(hashedUserId) {