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,