feat(exam-prep F8): слабые темы на дашборде + strategy=weak в тренажёре

backend/src/routes/exam-prep.js

@@ -317,6 +317,64 @@ const SQL = {
       FROM exam_topics
      WHERE exam_key = ? AND slug = ?
   `),
+
+  /* ── Weak topics (F8) ────────────────────────────────────────
+     Subtopics where the user has ≥3 attempts and accuracy < 60%.
+     Sorted by (accuracy ASC, attempts DESC). */
+  weakTopics: db.prepare(`
+    SELECT
+      tp.slug, tp.title, tp.parent_slug,
+      COUNT(a.id) AS attempts,
+      COALESCE(SUM(CASE WHEN a.is_correct = 1 THEN 1 ELSE 0 END), 0) AS correct,
+      COUNT(DISTINCT t.id) AS total_tasks,
+      COUNT(DISTINCT CASE WHEN a.is_correct = 1 THEN a.exam_task_id END) AS solved_tasks
+      FROM exam_topics tp
+      JOIN exam_tasks t ON t.subtopic = tp.slug AND t.exam_key = tp.exam_key
+      JOIN exam_attempts a ON a.exam_task_id = t.id
+                          AND a.user_id = ?
+                          AND a.is_correct IS NOT NULL
+     WHERE tp.exam_key = ? AND tp.parent_slug IS NOT NULL
+     GROUP BY tp.slug
+    HAVING attempts >= 3
+       AND (CAST(correct AS REAL) / attempts) < 0.6
+     ORDER BY (CAST(correct AS REAL) / attempts) ASC, attempts DESC
+     LIMIT 3
+  `),
+
+  weakTopicSlugs: db.prepare(`
+    SELECT tp.slug
+      FROM exam_topics tp
+      JOIN exam_tasks t ON t.subtopic = tp.slug AND t.exam_key = tp.exam_key
+      JOIN exam_attempts a ON a.exam_task_id = t.id AND a.user_id = ? AND a.is_correct IS NOT NULL
+     WHERE tp.exam_key = ? AND tp.parent_slug IS NOT NULL
+     GROUP BY tp.slug
+    HAVING COUNT(a.id) >= 3
+       AND (CAST(SUM(CASE WHEN a.is_correct = 1 THEN 1 ELSE 0 END) AS REAL) / COUNT(a.id)) < 0.6
+     ORDER BY (CAST(SUM(CASE WHEN a.is_correct = 1 THEN 1 ELSE 0 END) AS REAL) / COUNT(a.id)) ASC,
+              COUNT(a.id) DESC
+     LIMIT 3
+  `),
+
+  /* Practice batch — WEAK strategy: pick random tasks from top-3 weak subtopics,
+     skipping tasks the user has already solved correctly. */
+  weakBatchTasks: (examKey, slugs, userId, count) => {
+    if (!slugs.length) return [];
+    const ph = slugs.map(() => '?').join(',');
+    return db.prepare(`
+      SELECT id, task_idx, variant, task_type, text_html, figure_html, opts_json,
+             answer, solution_html, topic, subtopic, difficulty
+        FROM exam_tasks t
+       WHERE t.exam_key = ?
+         AND t.subtopic IN (${ph})
+         AND t.task_type IN ('mc','open')
+         AND NOT EXISTS (
+           SELECT 1 FROM exam_attempts a
+            WHERE a.exam_task_id = t.id AND a.user_id = ? AND a.is_correct = 1
+         )
+       ORDER BY RANDOM()
+       LIMIT ?
+    `).all(examKey, ...slugs, userId, count);
+  },
 };
 
 /* ── GET /api/exam-prep/tracks ──
@@ -435,11 +493,26 @@ router.get('/:examKey/practice/next', (req, res) => {
   let count = Number(req.query.count) || 10;
   count = Math.max(1, Math.min(count, 30));
 
-  const strategy = req.query.strategy === 'unsolved' ? 'unsolved' : 'random';
+  const strategyRaw = req.query.strategy;
+  let strategy = ['unsolved', 'weak', 'random'].includes(strategyRaw) ? strategyRaw : 'random';
   let rows;
-  if (strategy === 'unsolved') {
+  let weakSlugs = null;
+
+  if (strategy === 'weak') {
+    weakSlugs = SQL.weakTopicSlugs.all(req.user.id, examKey).map(r => r.slug);
+    if (weakSlugs.length === 0) {
+      // No weak topics yet → fall back to unsolved so the button still works
+      rows = SQL.practiceUnsolved.all(examKey, req.user.id, count);
+      strategy = 'unsolved-fallback';
+    } else {
+      rows = SQL.weakBatchTasks(examKey, weakSlugs, req.user.id, count);
+      if (!rows.length) {
+        // weak topics exist but all unsolved tasks exhausted → fallback to any from those topics
+        rows = SQL.weakBatchTasks(examKey, weakSlugs, -1 /* never matches */, count);
+      }
+    }
+  } else if (strategy === 'unsolved') {
     rows = SQL.practiceUnsolved.all(examKey, req.user.id, count);
-    // Fallback: if user has solved everything, supply random anyway so UX isn't a dead-end.
     if (!rows.length) rows = SQL.practiceRandom.all(examKey, count);
   } else {
     rows = SQL.practiceRandom.all(examKey, count);
@@ -447,7 +520,8 @@ router.get('/:examKey/practice/next', (req, res) => {
 
   res.json({
     strategy,
-    session_id: Date.now(),                  // ephemeral grouping key for these N attempts
+    weak_slugs: weakSlugs,
+    session_id: Date.now(),
     tasks: rows.map(shapeTask),
   });
 });
@@ -556,6 +630,7 @@ router.get('/:examKey/dashboard', (req, res) => {
   const recent = SQL.recentAttempts.all(req.user.id, examKey, 8);
   const heat   = SQL.activityHeatmap.all(req.user.id, examKey, twentyEightDaysAgo);
   const mocks  = SQL.recentMocks.all(req.user.id, examKey, 3);
+  const weak   = SQL.weakTopics.all(req.user.id, examKey);
 
   res.json({
     streak,
@@ -564,6 +639,16 @@ router.get('/:examKey/dashboard', (req, res) => {
       correct:  acc7.correct,
       pct:      acc7.attempts ? Math.round((acc7.correct / acc7.attempts) * 100) : null,
     },
+    weak_topics: weak.map(w => ({
+      slug:     w.slug,
+      title:    w.title,
+      parent:   w.parent_slug,
+      attempts: w.attempts,
+      correct:  w.correct,
+      accuracy: Math.round((w.correct / w.attempts) * 100),
+      total_tasks:  w.total_tasks,
+      solved_tasks: w.solved_tasks,
+    })),
     recent_attempts: recent.map(r => ({
       task_id:   r.exam_task_id,
       variant:   r.variant,

frontend/css/exam-prep.css

@@ -800,6 +800,49 @@
   width: 12px; height: 12px; aspect-ratio: 1;
 }
 
+/* Weak topics widget */
+.dh-weak-rows { display: flex; flex-direction: column; gap: 8px; }
+.dh-weak-row {
+  display: grid;
+  grid-template-columns: 1fr auto auto;
+  align-items: center;
+  gap: 12px;
+  padding: 11px 14px;
+  border: 1.5px solid var(--border);
+  border-radius: 10px;
+  text-decoration: none;
+  color: var(--text);
+  background: var(--surface);
+  transition: all .15s;
+}
+.dh-weak-row:hover {
+  border-color: #E63946;
+  background: rgba(230,57,70,.04);
+}
+.dh-weak-info { min-width: 0; }
+.dh-weak-title {
+  font-family: 'Unbounded', sans-serif; font-size: .9rem; font-weight: 700;
+}
+.dh-weak-meta {
+  font-size: .75rem; color: var(--text-3); margin-top: 2px;
+}
+.dh-weak-accuracy {
+  font-family: 'Unbounded', sans-serif; font-size: 1rem; font-weight: 800;
+  color: #E63946;
+  padding: 4px 10px; border-radius: 7px;
+  background: rgba(230,57,70,.10);
+}
+.dh-weak-cta {
+  display: inline-flex; align-items: center; gap: 4px;
+  font-size: .78rem; font-weight: 700; color: var(--violet);
+  white-space: nowrap;
+}
+.dh-weak-cta svg { width: 13px; height: 13px; }
+@media (max-width: 540px) {
+  .dh-weak-row { grid-template-columns: 1fr auto; }
+  .dh-weak-cta { display: none; }
+}
+
 /* Recent mocks rows */
 .dh-mock-row {
   display: grid;

frontend/js/exam-prep/dashboard.js

@@ -104,9 +104,9 @@
       </div>
     </div>
 
-    <div class="ep-card" style="opacity:.7">
+    <div class="ep-card" id="dh-weak">
       <h3>Слабые темы</h3>
-      <div class="ep-card-hint">Топ-3 темы с худшей точностью появятся после F6 (тегирование) и F8.</div>
+      <div id="dh-weak-list"><div class="ep-empty" style="padding:20px"><span class="ep-skel" style="width:200px;height:1em">&nbsp;</span></div></div>
     </div>
   `;
 
@@ -120,6 +120,7 @@
     renderRecent(dash.recent_attempts, track.exam_key);
     renderHeatmap(dash.heatmap);
     renderMocks(dash.recent_mocks, track.exam_key);
+    renderWeakTopics(dash.weak_topics || [], track.exam_key);
     if (window.lucide) lucide.createIcons();
   }
 
@@ -397,6 +398,37 @@ function formatRuDate(iso) {
   return `${d} ${months[m - 1]} ${y}`;
 }
 
+function renderWeakTopics(items, examKey) {
+  const el = document.getElementById('dh-weak-list');
+  if (!el) return;
+  if (!items.length) {
+    el.innerHTML = `<div class="ep-empty" style="padding:20px">
+      <i data-lucide="check-circle-2"></i>
+      <p>Слабых тем не выявлено. Решите больше задач, чтобы увидеть приоритеты.</p>
+    </div>`;
+    return;
+  }
+  el.innerHTML = `
+    <div class="dh-weak-rows">
+      ${items.map(w => {
+        const tasksLeft = Math.max(0, w.total_tasks - w.solved_tasks);
+        return `<a class="dh-weak-row" href="/exam-prep/${examKey}/topics/${encodeURIComponent(w.slug)}">
+          <div class="dh-weak-info">
+            <div class="dh-weak-title">${escapeHtml(w.title)}</div>
+            <div class="dh-weak-meta">${w.correct}/${w.attempts} попыток · ${tasksLeft} задач не взято</div>
+          </div>
+          <div class="dh-weak-accuracy">${w.accuracy}%</div>
+          <div class="dh-weak-cta">Прокачать <i data-lucide="arrow-right"></i></div>
+        </a>`;
+      }).join('')}
+    </div>
+    <div class="ep-cta-row" style="margin-top:14px">
+      <a class="ep-btn ep-btn-primary" href="/exam-prep/${examKey}/practice?strategy=weak">
+        <i data-lucide="target"></i> Тренировать слабые темы
+      </a>
+    </div>`;
+}
+
 function renderMocks(items, examKey) {
   const el = document.getElementById('dh-mocks-list');
   if (!el) return;

frontend/js/exam-prep/practice.js

@@ -13,11 +13,16 @@
 
   // Per-session state
   let batch        = null;     // { strategy, session_id, tasks: [...] }
-  let strategy     = readPersistedStrategy() || 'random';
+  let strategy     = readStrategyFromUrl() || readPersistedStrategy() || 'random';
   let count        = readPersistedCount();
   let finalized    = false;
   const results    = new Map();   // taskId -> { isCorrect: 0|1|null, attempts: number }
 
+  function readStrategyFromUrl() {
+    const m = location.search.match(/[?&]strategy=(random|unsolved|weak)/);
+    return m ? m[1] : null;
+  }
+
   /* ── Strategy persistence (local pref) ──────────────────────── */
   function readPersistedStrategy() {
     try { return localStorage.getItem(`exam_prep_${examKey}_practice_strategy`); } catch { return null; }
@@ -52,7 +57,8 @@
             <button class="pr-strategy-btn ${strategy === 'unsolved' ? 'active' : ''}" data-strat="unsolved">
               <i data-lucide="circle-dashed"></i> Только нерешённые
             </button>
-            <button class="pr-strategy-btn disabled" disabled title="Появится в F8 после тегирования тем">
+            <button class="pr-strategy-btn ${strategy === 'weak' ? 'active' : ''}" data-strat="weak"
+                    title="Топ-3 темы с худшей точностью (нужно ≥3 попыток в теме)">
               <i data-lucide="target"></i> Слабые темы
             </button>
           </div>
@@ -192,7 +198,7 @@
         </div>
         <div class="pr-summary-stat">
           <div class="ep-stat-label">Стратегия</div>
-          <div class="ep-stat-value" style="font-size:1.1rem">${strategy === 'unsolved' ? 'Нерешённые' : 'Случайные'}</div>
+          <div class="ep-stat-value" style="font-size:1.1rem">${strategyLabel(strategy)}</div>
         </div>
       </div>
       <div class="ep-cta-row">
@@ -233,4 +239,11 @@
   function escapeHtml(s) {
     return String(s || '').replace(/[&<>"']/g, c => ({ '&':'&amp;', '<':'&lt;', '>':'&gt;', '"':'&quot;', "'":'&#39;' }[c]));
   }
+
+  function strategyLabel(s) {
+    if (s === 'unsolved') return 'Нерешённые';
+    if (s === 'weak')     return 'Слабые темы';
+    if (s === 'unsolved-fallback') return 'Нерешённые (fallback)';
+    return 'Случайные';
+  }
 })();