feat(exam): Phase 4 — контроллер использует task-level textbook_slug/paragraph

- SELECT добавляет t.textbook_slug, t.textbook_paragraph во все 7 запросов
  (getVariantTasks, practiceRandom, practiceUnsolved, pickRandomByDifficulty,
   topicTasksUnsolved/Any, weakBatchTasks, getTasksByIds)
- shapeTask() предпочитает task-level ссылку, fallback на refMap subtopic
- /variants/:n/tasks аналогично использует per-task поля
- getParaTitle() строит map chapter:para -> title из g9_textbook_sections.json

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/routes/exam-prep.js

@@ -73,7 +73,7 @@ const SQL = {
   getVariantTasks: db.prepare(`
     SELECT
       id, task_idx, task_type, text_html, figure_html, opts_json,
-      answer, solution_html, topic, subtopic
+      answer, solution_html, topic, subtopic, textbook_slug, textbook_paragraph
       FROM exam_tasks
      WHERE exam_key = ? AND variant = ?
      ORDER BY task_idx
@@ -87,7 +87,7 @@ const SQL = {
   practiceRandom: db.prepare(`
     SELECT
       id, task_idx, variant, task_type, text_html, figure_html, opts_json,
-      answer, solution_html, topic, subtopic, difficulty
+      answer, solution_html, topic, subtopic, difficulty, textbook_slug, textbook_paragraph
       FROM exam_tasks
      WHERE exam_key = ?
        AND task_type IN ('mc','open')
@@ -116,7 +116,8 @@ const SQL = {
   practiceUnsolved: db.prepare(`
     SELECT
       t.id, t.task_idx, t.variant, t.task_type, t.text_html, t.figure_html,
-      t.opts_json, t.answer, t.solution_html, t.topic
+      t.opts_json, t.answer, t.solution_html, t.topic, t.subtopic, t.difficulty,
+      t.textbook_slug, t.textbook_paragraph
       FROM exam_tasks t
      WHERE t.exam_key = ?
        AND t.task_type IN ('mc','open')
@@ -168,7 +169,7 @@ const SQL = {
     const ph = ids.map(() => '?').join(',');
     return db.prepare(`
       SELECT id, task_idx, variant, task_type, text_html, figure_html, opts_json,
-             answer, solution_html, topic
+             answer, solution_html, topic, subtopic, textbook_slug, textbook_paragraph
         FROM exam_tasks
        WHERE id IN (${ph})
     `).all(...ids);
@@ -320,7 +321,7 @@ const SQL = {
      correctly, when ?exclude_solved=1. */
   topicTasksUnsolved: db.prepare(`
     SELECT id, task_idx, variant, task_type, text_html, figure_html, opts_json,
-           answer, solution_html, topic, subtopic, difficulty
+           answer, solution_html, topic, subtopic, difficulty, textbook_slug, textbook_paragraph
       FROM exam_tasks t
      WHERE t.exam_key = ? AND t.subtopic = ?
        AND t.task_type IN ('mc','open')
@@ -334,7 +335,7 @@ const SQL = {
 
   topicTasksAny: db.prepare(`
     SELECT id, task_idx, variant, task_type, text_html, figure_html, opts_json,
-           answer, solution_html, topic, subtopic, difficulty
+           answer, solution_html, topic, subtopic, difficulty, textbook_slug, textbook_paragraph
       FROM exam_tasks t
      WHERE t.exam_key = ? AND t.subtopic = ?
        AND t.task_type IN ('mc','open')
@@ -392,7 +393,7 @@ const SQL = {
     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
+             answer, solution_html, topic, subtopic, difficulty, textbook_slug, textbook_paragraph
         FROM exam_tasks t
        WHERE t.exam_key = ?
          AND t.subtopic IN (${ph})
@@ -480,19 +481,33 @@ router.get('/:examKey/variants/:n/tasks', (req, res) => {
   if (!rows.length) return res.status(404).json({ error: 'Variant not found or empty' });
 
   const refMap = getTopicRefMap(examKey);
-  const tasks = rows.map(r => ({
+  const tasks = rows.map(r => {
-    id:        r.id,
+    let topic_ref = null;
-    idx:       r.task_idx,
+    if (r.textbook_slug) {
-    type:      r.task_type,
+      const paraTitle = getParaTitle(r.textbook_slug, r.textbook_paragraph);
-    text:      r.text_html,
+      const fallbackTitle = r.subtopic ? (refMap.get(r.subtopic)?.title || null) : null;
-    figure:    r.figure_html,
+      topic_ref = {
-    opts:      r.opts_json ? safeJson(r.opts_json) : null,
+        title:     paraTitle || fallbackTitle,
-    answer:    r.answer,
+        slug:      r.textbook_slug,
-    solution:  r.solution_html,
+        paragraph: r.textbook_paragraph ?? null,
-    topic:     r.topic,
+      };
-    subtopic:  r.subtopic,
+    } else if (r.subtopic) {
-    topic_ref: r.subtopic ? (refMap.get(r.subtopic) || null) : null,
+      topic_ref = refMap.get(r.subtopic) || null;
-  }));
+    }
+    return {
+      id:        r.id,
+      idx:       r.task_idx,
+      type:      r.task_type,
+      text:      r.text_html,
+      figure:    r.figure_html,
+      opts:      r.opts_json ? safeJson(r.opts_json) : null,
+      answer:    r.answer,
+      solution:  r.solution_html,
+      topic:     r.topic,
+      subtopic:  r.subtopic,
+      topic_ref,
+    };
+  });
   res.json({ variant: n, tasks });
 });
 
@@ -520,7 +535,20 @@ function getTopicRefMap(examKey) {
 }
 
 function shapeTask(r, refMap) {
-  const ref = (refMap && r.subtopic) ? refMap.get(r.subtopic) : null;
+  // Prefer task-level textbook over subtopic-level fallback.
+  let topic_ref = null;
+  if (r.textbook_slug) {
+    // Build title from para map, fall back to subtopic title via refMap.
+    const fallbackTitle = (refMap && r.subtopic) ? (refMap.get(r.subtopic)?.title || null) : null;
+    const paraTitle = getParaTitle(r.textbook_slug, r.textbook_paragraph);
+    topic_ref = {
+      title:     paraTitle || fallbackTitle,
+      slug:      r.textbook_slug,
+      paragraph: r.textbook_paragraph ?? null,
+    };
+  } else if (refMap && r.subtopic) {
+    topic_ref = refMap.get(r.subtopic) || null;
+  }
   return {
     id:       r.id,
     idx:      r.task_idx,
@@ -534,10 +562,35 @@ function shapeTask(r, refMap) {
     topic:    r.topic ?? null,
     subtopic: r.subtopic ?? null,
     difficulty: r.difficulty ?? null,
-    topic_ref: ref || null,
+    topic_ref: topic_ref || null,
   };
 }
 
+/* ── Para-title cache ─────────────────────────────────────────────
+   chapter_slug + paragraph_int → title string from g9_textbook_sections.json.
+   Built once on first call; covers all books 5-9. */
+let _paraCache = null;
+function _buildParaCache() {
+  const cache = new Map();
+  try {
+    const sections = require('../../scripts/data/g9_textbook_sections');
+    for (const s of sections) {
+      if (!s.para_id || !s.title) continue;
+      const m = String(s.para_id).match(/^p(\d+)$/);
+      if (!m) continue;
+      const n = parseInt(m[1], 10);
+      cache.set(`${s.chapter_slug}:${n}`, s.title);
+    }
+  } catch { /* if file missing, silently skip */ }
+  return cache;
+}
+
+function getParaTitle(chapterSlug, paraNum) {
+  if (!chapterSlug || paraNum == null) return null;
+  if (!_paraCache) _paraCache = _buildParaCache();
+  return _paraCache.get(`${chapterSlug}:${paraNum}`) || null;
+}
+
 /* ── Difficulty distribution for "Random" practice ───────────────
    Returns an array [n1,n2,n3,n4,n5] of how many tasks of each
    difficulty level (1..5) to pick, summing to N. Position 1 of the
@@ -583,7 +636,7 @@ function pickRandomByDifficulty(examKey, count, excludeSlugs) {
     if (limit === 0) continue;
     const sql = `
       SELECT id, task_idx, variant, task_type, text_html, figure_html, opts_json,
-             answer, solution_html, topic, subtopic, difficulty
+             answer, solution_html, topic, subtopic, difficulty, textbook_slug, textbook_paragraph
         FROM exam_tasks
        WHERE exam_key = ? AND task_type IN ('mc','open')
          AND difficulty = ?