Sync from GitHub via hub-sync

[NAH-36] Custom Recall frontend (gr.Server) + JSON API over the backend (#17)

[NAH-32] difficulty toggle UI: make-it-harder / make-it-easier (#14)

Merge pull request #4 from francisco-magana/hygienepile-0k/nah-13-weak-topic-targeting-bias-queue-toward-low-grade-topics

[NAH-13] Weak-topic targeting: bias next_card toward low-grade topics

[NAH-8] grade_answer: strict-JSON grading + parser/repair retry

Add app, content pipeline, learning engine, and gitignore