feat(api): add endpoint to fetch all assigned cards with related data

- Implemented GET /api/cards/assigned endpoint to return all cards assigned to the current user (as member or author)
- Included related entities: labels, task lists, tasks, attachments, custom fields, memberships, and users
- Added route to config/routes.js
- Ensured response format matches project conventions for proper client display

server/config/routes.js

@@ -227,6 +227,8 @@ module.exports.routes = {
     skipAssets: false,
   },
 
+  'GET /api/cards/assigned': 'cards/assigned',
+
   'GET /*': {
     view: 'index',
     skipAssets: true,