Persistent storage for all flashcard decks with spaced repetition metadata. JSON structure with decks keyed by slug (lowercase, hyphenated name). Each deck contains: name, created date, source description, and cards array. Each card tracks: id, front, back, created, lastReviewed, nextReview, interval (days until next review), correctCount, incorrectCount. New cards start with interval=1. On correct: interval doubles. On incorrect: interval resets to 1. Don't store session/temporary review data here—only committed deck state.