feat(gamification): Phase 1 — full kill-switch + textbook XP wrapping

Until now the 'gamification' feature flag did nothing: it had no row in
app_settings, the admin couldn't toggle it, awardXP/awardCoins ignored
it, and the CSS only hid three dashboard widgets — XP bars in textbooks
stayed visible regardless.

Phase 1 closes every hole.

Backend (source of truth):
  • migration 029 seeds feature_gamification_enabled=1
  • new isGamificationEnabled() helper in gamification/_shared.js with a
    30s cache + invalidateGamificationCache() for instant admin toggles
  • awardXP / awardCoins / updateStreak / unlockAchievement /
    checkAchievements all bail out when the flag is off
  • /api/gamification/* and /api/shop/* (user routes) return 404 when
    disabled; admin routes remain open so the switch itself is reachable
  • adminController.updateFeatures gains 'gamification' in the allow-list
    and invalidates the cache on flip

Frontend:
  • LS.isGamificationEnabled() (synchronous, populated by loadFeatures)
    so xp.js + applyCosmetics can bail without a round-trip
  • xp.js load/add/flush become no-ops when the flag is off
  • applyCosmetics skips the round-trip when off
  • CSS .no-gamification rule expanded to cover .hero-xp-badge, .po-xp,
    .xp-card, .xp-bar, #frames-section, and a universal [data-gamified]
    hook for future blocks

Textbooks (Variant 2 of the plan):
  • backend/scripts/wrap_textbook_xp.py — idempotent script that adds
    data-gamified to 167 XP tags across 63 textbook files (chapters +
    hubs, all subjects/grades). Single CSS rule now hides everything.

Verified end-to-end: with the flag off, awardXP/awardCoins write nothing;
flipping back restores normal behavior.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/db/migrations/029_gamification_feature_flag.sql

@@ -0,0 +1,15 @@
+-- ═══════════════════════════════════════════════════════════════
+-- 029: Seed the gamification feature flag
+--
+-- Until now there was no row for feature_gamification_enabled in
+-- app_settings — meaning /api/features returned `gamification: undefined`
+-- and the front-end kill-switch (`body.no-gamification`) never engaged,
+-- regardless of admin intent.
+--
+-- Default = ON. Admins can disable via PATCH /api/admin/features
+-- once 'gamification' is added to the allowed list (handled in
+-- adminController.js, not this migration).
+-- ═══════════════════════════════════════════════════════════════
+
+INSERT OR IGNORE INTO app_settings (key, value)
+  VALUES ('feature_gamification_enabled', '1');