Complete per-user data isolation, bug fixes, and Admin wipe trigger

Gracefully disable YouTube in Cloud Mode

feat: use Invidious API for YouTube transcripts

feat: browser-side YouTube transcript fetching

fix: pre-download embedding model during Docker build to avoid DNS failures on HF Spaces

Add health check endpoint

Add multi-user support with session-based student IDs

Fix: Implement automatic date/day progression system

Add data persistence and comprehensive citation system

Major improvements: Multi-subject plan generation, enhanced quiz/lesson quality, fixed UI layout

feat: Implement AI Plan Generator (Lite Mode) and Focus Mode Layout

Update backend RAG logic, app UI, and add documentation

Initial commit of FocusFlow backend