[B4] Challenges, mini-games and easter eggs

Backlog. Everything opt-in and playful that rewards XP.

Depends on: [G1] XP ledger.

Ideas parked here

• Challenges. A challenge is an objective plus a completion rule ("12 countries in 12 months", "one film per decade", "clear these 20 games"). Reward is XP, so it plugs straight into the level system. Implementation option to evaluate: a challenge is essentially a List + a rule, rather than a new primitive. Shape: one shared monthly challenge for everyone, plus one extra actionable challenge reserved for premium (see [B9]).
• Daily mini-game — a Wordle-style media guessing game or a logic puzzle. This replaces the idea of daily quests, which is rejected: "rate something today" is pure habit manufacturing, disconnected from actually watching or reading anything, and it pushes users to log junk that pollutes the data.
• Easter eggs, some of them unlocking secret achievements (the secret mechanic itself is built in [G2]/[G3]).
• "Séance à l'aveugle": the app proposes a work deliberately outside your habits; watching it earns XP. Discovery and play in one gesture, but it depends on a recommendation engine that does not exist yet — far out.

Challenges are not a near-term priority.