Rewrite Medium and Hard tasks for proper difficulty curve

Fix 3 critical bugs: trailing-dot grading, hard task difficulty, invalid task_id crash, add IP to easy task description

Fix openenv validate: add uv.lock, project.scripts, openenv-core dep, main() function

Add stunning HTML landing page with live demo, task cards, and reward visualization

Add root endpoint to fix Not Found on Space homepage

DNS-Env: DNS Zone File Debugging Environment for OpenEnv