feat(gamification): Phase 4 — standalone coin events + coin_log

Coins were always 1:10 of XP. Now they have their own event log + a
helper that dedups by reason within a configurable window.

Backend:
  • migration 032 creates coin_log (user_id, amount, reason, created_at)
    with indices for the 'fired today?' check
  • awardCoins now records into coin_log on every call (reason defaults
    to 'xp_bonus' for the legacy XP-proportional path)
  • awardCoinsOnce(userId, amount, reason, window) — fires the bonus
    only if no row matches in the window:
      'day'     → DATE(created_at) = today
      'week'    → ISO week match
      'forever' → never twice

Wired events (Phase 4 subset of the plan):
  • Daily login — 10 coins, once/day. Hooked in updateStreak so the
    bonus rides on the existing 'daily_activity' XP trigger.
  • Daily goal completion — 15/25/40 coins (easy/medium/hard), once/day.
    Sits next to the existing tier XP bonus in updateDailyGoal.
  • Variant clear — 30 coins, once per (user, variant) forever. Fires
    from the exam-prep attempts endpoint when the user's final correct
    answer fills out a math9 variant.

Deferred (need invasive trigger hooks): weekly goal, paragraph close,
boss defeated, referral.

Verified end-to-end: awardCoinsOnce returns true→false on repeated
calls, coin_log records the first, coins balance moves once.

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

backend/src/controllers/gamification/service.js

@@ -14,10 +14,45 @@ const {
    ═══════════════════════════════════════════════════════════════════════ */
 
 /* ── Coins ─────────────────────────────────────────────────────────── */
-function awardCoins(userId, amount /*, reason */) {
+function awardCoins(userId, amount, reason) {
   if (!amount || amount <= 0) return;
   if (!isGamificationEnabled()) return;       // master kill-switch
   stmts.incrCoins.run(amount, userId);
+  // Always record into coin_log for traceability + the upcoming
+  // "coin history" UI. Old callers that omitted reason fall back to
+  // the generic 'xp_bonus' tag (since awardXP forwards its own reason).
+  try {
+    db.prepare('INSERT INTO coin_log (user_id, amount, reason) VALUES (?, ?, ?)')
+      .run(userId, amount, reason || 'xp_bonus');
+  } catch { /* coin_log table missing — pre-032 install */ }
+}
+
+/* awardCoinsOnce — same as awardCoins but only fires if no row with
+   the given reason already exists in the dedup window.
+     window = 'day'     → only once per UTC calendar day
+              'week'    → only once per ISO week (Mon-Sun)
+              'forever' → only once ever
+   Returns true if the bonus was awarded, false if it was deduped. */
+function awardCoinsOnce(userId, amount, reason, window = 'day') {
+  if (!amount || amount <= 0) return false;
+  if (!isGamificationEnabled()) return false;
+  try {
+    let whereClause;
+    if (window === 'forever') {
+      whereClause = '';
+    } else if (window === 'week') {
+      // ISO week — same %Y-%W as sqlite's strftime.
+      whereClause = "AND strftime('%Y-%W', created_at) = strftime('%Y-%W', 'now')";
+    } else { // 'day'
+      whereClause = "AND DATE(created_at) = DATE('now')";
+    }
+    const exists = db.prepare(`
+      SELECT 1 FROM coin_log WHERE user_id = ? AND reason = ? ${whereClause} LIMIT 1
+    `).get(userId, reason);
+    if (exists) return false;
+  } catch { /* table missing — proceed without dedup so coins still arrive */ }
+  awardCoins(userId, amount, reason);
+  return true;
 }
 
 /* ── XP ────────────────────────────────────────────────────────────── */
@@ -76,6 +111,8 @@ function updateStreak(userId) {
   stmts.setStreak.run(newStreak, newBest, today, userId);
 
   awardXP(userId, 30, 'daily_activity');
+  // Daily-login coin — once per day, on top of the streak XP.
+  awardCoinsOnce(userId, 10, 'daily_login', 'day');
   return newStreak;
 }
 
@@ -501,8 +538,13 @@ function updateDailyGoal(userId, addTests, addXp) {
     const already = stmts.checkGoalBonus.get(userId, today);
     if (!already) {
       const pref = stmts.getUserGoalTier.get(userId);
-      const tier = GOAL_TIERS[(pref && pref.goal_tier) || 'medium'] || GOAL_TIERS.medium;
+      const tierKey = (pref && pref.goal_tier) || 'medium';
+      const tier = GOAL_TIERS[tierKey] || GOAL_TIERS.medium;
       awardXP(userId, tier.bonus, 'daily_goal');
+      // Standalone coin bonus on top of XP: easy=15, medium=25, hard=40.
+      // Once-per-day dedup is implicit (checkGoalBonus already guards XP).
+      const coinByTier = { easy: 15, medium: 25, hard: 40 };
+      awardCoinsOnce(userId, coinByTier[tierKey] || 25, 'daily_goal', 'day');
       try {
         sse.emit(userId, { type: 'daily_goal', message: `Дневная цель выполнена! +${tier.bonus} XP`, icon: 'target' });
       } catch (e) { console.error('[daily_goal]', e.message); }
@@ -605,7 +647,7 @@ function updateChallenges(userId, score, total, subjectSlug, topicId) {
 
 module.exports = {
   // Core
-  awardXP, awardCoins, getXPInfo, updateStreak,
+  awardXP, awardCoins, awardCoinsOnce, getXPInfo, updateStreak,
   // Achievements
   seedAchievements, unlockAchievement, checkAchievements, checkRedBookAchievements, pushAchievementNotif,
   // Hooks (for other controllers)