[G3] MVP achievement catalogue, wording and badge artwork

The content side of the achievement system: what exists, what it is called, what it looks like. Deliberately split from the engine ([G2]) because this is catalogue + copy + artwork work, not infrastructure.

Depends on: [G2] Achievement engine.

Scope

• Catalogue definition, MVP families:

  • Volume milestones, per domain and per consumption type (episodes / movies / finished series / books / games / albums) with thresholds calibrated separately for each — 100 episodes is not 100 finished series.
  • Ritual: 7/30/365 consecutive days, marathon (several episodes the same day), late-night viewing (watchedAt carries the time, User.timezone gives the local hour).
  • Exploration: N distinct decades, N genres, N countries, one title in each domain. Existing helpers: decade.util.ts, rating-by-group.util.ts, genres.
  • Completion, aggregate-only: 50 series watched in total, 5 series of 5+ seasons finished, ratio of started/finished held over N titles. Never "one series finished = one achievement".
  • Seasonal/temporal: horror in October, a film released in your birth year (User.birthDate exists).
  • A few secret entries, plus the two distinct families secrets and humour (they are not the same thing).
  • Profile completion and onboarding completion achievements (the latter pairs with [G8]).
• "Significant day" threshold per domain for streaks: one episode or one movie, N pages, N minutes played, one album. Known limitation to work around: real playtime is not measured outside the Steam import, and book progress depends on what the user enters — the threshold must be based on what is actually measured.
• Which achievements an import may unlock, and which are excluded.
• i18n: a dedicated gamification.json per locale rather than growing common/other. Roughly 2 strings per entry per locale — budget it.
• Badge artwork. Real design work, not a detail: an icon system for ~60 entries in the "Séance" register (apps/web/DESIGN.md), plus how tiers and locked/secret states are rendered. To be settled here so [G5] only has to display it.

Out of scope

Engine mechanics (G2), screens (G5).