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>

frontend/textbooks/algebra_8_ch2.html

@@ -373,7 +373,7 @@ input,select,textarea{font-family:inherit}
           <div class="hp-bar"><div id="hero-hp-fill" class="hp-fill"></div></div>
           <span id="hero-hp-text" class="hp-text">0%</span>
         </div>
-        <div id="hero-xp-badge" class="hero-xp-badge" title="Опыт"></div>
+        <div id="hero-xp-badge" class="hero-xp-badge" title="Опыт" data-gamified></div>
       </div>
     </section>
 
@@ -764,8 +764,8 @@ function buildSidebar(id){
   const xpInLv  = STATE.xp - xpForLv;
   const xpRange = xpNext - xpForLv;
   const xpPct   = xpRange > 0 ? Math.round(xpInLv / xpRange * 100) : 100;
-  html += `<div class="xp-card">
-    <div class="xp-card-title">
+  html += `<div class="xp-card" data-gamified>
+    <div class="xp-card-title" data-gamified>
       <span>XP-прогресс</span>
       <span class="xp-level">Ур. ${STATE.level}</span>
     </div>