Fix upload directory path issue

- Change UPLOAD_DIR from ../uploads to ./uploads in server/.env
- Use absolute path with process.cwd() in upload middleware
- Ensures uploads directory is created in correct location

.kiro/specs/beach-litter-mapping/tasks.md

@@ -145,7 +145,7 @@
 
 ## Cleanup Tracking System
 
-- [-] 18. Implement "Mark as Cleaned" functionality
+- [x] 18. Implement "Mark as Cleaned" functionality
   - Add `cleaned_at` timestamp field to reports table
   - Add `cleaned_by_user_id` field to track who marked it as cleaned
   - Create POST /api/reports/:id/mark-cleaned backend endpoint