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/geometry_8_ch2.html

@@ -262,7 +262,7 @@ a{color:inherit;text-decoration:none}
           <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>
 
@@ -432,7 +432,7 @@ function buildSidebar(id){
   const box=document.getElementById('sidebar-content'); const sb=SIDEBARS[id]||SIDEBARS.p1; let html='';
   const xpForLv=_xpForLevel(STATE.level),xpNext=_xpForLevel(STATE.level+1);
   const xpInLv=STATE.xp-xpForLv,xpRange=xpNext-xpForLv,xpPct=xpRange>0?Math.round(xpInLv/xpRange*100):100;
-  html+=`<div class="xp-card"><div class="xp-card-title"><span>XP-прогресс</span><span class="xp-level">Ур. ${STATE.level}</span></div><div class="xp-bar"><div class="xp-fill" style="width:${xpPct}%"></div></div><div class="xp-nums"><span>${STATE.xp} XP</span><span>${xpNext} XP</span></div></div>`;
+  html+=`<div class="xp-card" data-gamified><div class="xp-card-title" data-gamified><span>XP-прогресс</span><span class="xp-level">Ур. ${STATE.level}</span></div><div class="xp-bar"><div class="xp-fill" style="width:${xpPct}%"></div></div><div class="xp-nums"><span>${STATE.xp} XP</span><span>${xpNext} XP</span></div></div>`;
   html+=`<div class="sidecard"><h4>${sb.title}</h4>`; sb.rows.forEach(([k,v])=>{ html+=`<div class="sidecard-row"><b>${k}</b>${v?' — '+v:''}</div>`; }); html+='</div>';
   const tip=TIPS.find(t=>t.sec===id)||TIPS[0];
   html+=`<div class="sidecard" style="background:linear-gradient(135deg,var(--warn-bg,#fef3c7),var(--pri-soft));border-color:var(--warn,#f59e0b)"><h4 style="color:#065f46;display:flex;align-items:center;gap:6px"><svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" style="width:14px;height:14px"><rect x="3" y="3" width="18" height="14" rx="1"/></svg>Подсказка</h4><div class="sidecard-row" style="margin-bottom:0;font-size:.84rem;line-height:1.55">${tip.html}</div></div>`;