learning_cards/{id} → {
  title, subtitle, content, keyTakeaway,
  imageUrl,
  order,       ← GLOBAL sequential order (1, 2, 3 … N) — like quizzes
               ← NO dateKey — order-based, not date-based
  active,
  difficulty,  ← "beginner" | "intermediate" | "advanced"
  tags,        ← string[]
  usefulCount, notUsefulCount,
  createdAt, updatedAt
}

App behaviour:
  Free users  → see cards in order, 5 at a time
  After 5     → gate: watch ad (+5 more) or upgrade to Pro
  Pro users   → unlimited sequential cards

quizzes/{id} → {
  order, image_url, result_image_url, question,
  options[], correct_answer,
  explanation: { technical, psychological, lesson }
}

[{ "order":1, "image_url":"...", "question":"...", "options":["A","B","C","D"], "correct_answer":"A", "explanation":{"technical":"...","psychological":"...","lesson":"..."} }]