Add challenges 038 through 040

[Bmowville]

Summary

• add three advanced SQL challenges so the project reaches 40 total
• make the final challenge tail harder: stockout risk, trial conversion windows, and revenue leakage reconciliation
• update the README, learning paths, roadmap, site count, and Challenge Finder

Closes #7

Validation

• python scripts/validate_challenges.py --write-expected
• python scripts/validate_challenges.py
• node --check docs/challenge-finder.js
• git diff --check
• local browser check for 40 count and advanced finder results