feat: stages module, mobility frontend, theme toggle, employeeOnly access control

- Add stages module with full CRUD API and admin overview island
- Add mobility overview island (Liste, Kanban, Detail CRUD views)
- Add contract PDF upload/download endpoints for mobilites
- Add light/dark theme toggle in header
- Add employeeOnly flag to hide entire modules from students (admin, students, stages)
- Add read-only GET endpoints for modules/ues/ue-modules in notes module
- Add [slug].tsx catch-all routes for direct URL navigation
- Replace old mobility table with mobilites + stages schema (migration 0004)
- Allow students to create mobilites and upload contracts
- Redirect authenticated users from / to /apps catalog

compose.prod.yml

@@ -30,9 +30,12 @@ services:
     ports:
       - "4430:443"
     env_file: .env
+    volumes:
+      - contracts:/app/uploads/contracts
     depends_on:
       migrate:
         condition: service_completed_successfully
 
 volumes:
   db_data:
+  contracts: