feat(catalog): хаб-страница для Алгебры 8 (3 главы под единым слагом)

- migration 014: parent_slug column + algebra-8 hub row +
  rename old algebra-8 → algebra-8-ch1 (progress сохраняется
  через стабильный textbook_id=3)
- backend/routes/textbooks.js: GET / фильтрует parent_slug IS NULL;
  aggregated progress для хабов; новый GET /:slug/children
- algebra_8_hub.html: новая хаб-страница с 3 карточками глав,
  hero с общим прогрессом, XP-бейдж, ссылки на главы
- algebra_8/ch2/ch3: кнопки cross-chapter заменены на
  одну «К алгебре 8» в шапке

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/routes/textbooks.js

@@ -59,24 +59,66 @@ function checkAssignmentCompletion(userId, textbookId, readSet) {
    LITERAL ROUTES FIRST — must come before /:slug
    ════════════════════════════════════════════════ */
 
-/* GET /api/textbooks — list with current user's progress */
+/* GET /api/textbooks — catalog list (top-level only) with current user's progress.
+   Rows with a parent_slug are chapters hidden from the catalog but still directly accessible. */
 router.get('/', (req, res) => {
   const rows = db.prepare(`
     SELECT t.id, t.slug, t.subject, t.grade, t.title, t.author, t.description,
            t.html_path, t.para_count, t.color, t.sort_order
-    FROM textbooks t WHERE t.is_active = 1
+    FROM textbooks t WHERE t.is_active = 1 AND t.parent_slug IS NULL
     ORDER BY t.sort_order, t.subject, t.grade
   `).all();
+
   const myProgress = db.prepare(`
     SELECT textbook_id, paragraphs_read, last_para, last_at FROM textbook_progress WHERE user_id=?
   `).all(req.user.id);
-  const map = {};
+  const progressById = {};
   for (const p of myProgress) {
     let arr = [];
     try { arr = JSON.parse(p.paragraphs_read || '[]'); } catch {}
-    map[p.textbook_id] = { read: arr, last_para: p.last_para, last_at: p.last_at };
+    progressById[p.textbook_id] = { read: arr, last_para: p.last_para, last_at: p.last_at };
   }
-  res.json({ textbooks: rows.map(t => ({ ...t, progress: map[t.id] || { read: [], last_para: null, last_at: null } })) });
+
+  /* Prepared statements reused for each hub row that has children. */
+  const stmtChildren = db.prepare(`
+    SELECT id FROM textbooks WHERE parent_slug = ? AND is_active = 1
+  `);
+  const stmtChildProgress = db.prepare(`
+    SELECT textbook_id, paragraphs_read, last_para, last_at FROM textbook_progress
+    WHERE user_id = ? AND textbook_id IN (SELECT id FROM textbooks WHERE parent_slug = ?)
+    ORDER BY last_at DESC
+  `);
+
+  const textbooks = rows.map(t => {
+    const children = stmtChildren.all(t.slug);
+    if (children.length === 0) {
+      return { ...t, progress: progressById[t.id] || { read: [], last_para: null, last_at: null } };
+    }
+
+    /* Hub: aggregate progress across all child chapters. */
+    const childRows = stmtChildProgress.all(req.user.id, t.slug);
+    const readSet = new Set();
+    let last_para = null;
+    let last_at   = null;
+
+    for (const cp of childRows) {
+      let arr = [];
+      try { arr = JSON.parse(cp.paragraphs_read || '[]'); } catch {}
+      for (const item of arr) readSet.add(item);
+      /* childRows ordered by last_at DESC — first row is the most recent. */
+      if (last_para === null) {
+        last_para = cp.last_para;
+        last_at   = cp.last_at;
+      }
+    }
+
+    return {
+      ...t,
+      progress: { read: [...readSet], last_para, last_at },
+    };
+  });
+
+  res.json({ textbooks });
 });
 
 /* GET /api/textbooks/bookmarks/all — all my bookmarks across textbooks */
@@ -123,6 +165,43 @@ router.delete('/bookmarks/:id', (req, res) => {
    :slug ROUTES (catch-all per textbook)
    ════════════════════════════════════════════════ */
 
+/* GET /api/textbooks/:slug/children — chapter list for a hub textbook, with per-user progress.
+   Used by hub pages to render chapter cards and progress bars. */
+// @public-by-design: router-level authMiddleware (line 7) covers all routes in this file
+router.get('/:slug/children', (req, res) => {
+  const hub = db.prepare('SELECT id FROM textbooks WHERE slug=? AND is_active=1').get(req.params.slug);
+  if (!hub) return res.status(404).json({ error: 'Учебник не найден' });
+
+  const children = db.prepare(`
+    SELECT t.id, t.slug, t.title, t.description, t.html_path, t.para_count, t.sort_order, t.color
+    FROM textbooks t
+    WHERE t.parent_slug = ? AND t.is_active = 1
+    ORDER BY t.sort_order
+  `).all(req.params.slug);
+
+  const progressRows = db.prepare(`
+    SELECT textbook_id, paragraphs_read, last_para, last_at
+    FROM textbook_progress
+    WHERE user_id = ? AND textbook_id IN (
+      SELECT id FROM textbooks WHERE parent_slug = ? AND is_active = 1
+    )
+  `).all(req.user.id, req.params.slug);
+
+  const progMap = {};
+  for (const p of progressRows) {
+    let arr = [];
+    try { arr = JSON.parse(p.paragraphs_read || '[]'); } catch {}
+    progMap[p.textbook_id] = { read: arr, last_para: p.last_para, last_at: p.last_at };
+  }
+
+  res.json({
+    children: children.map(c => ({
+      ...c,
+      progress: progMap[c.id] || { read: [], last_para: null, last_at: null },
+    })),
+  });
+});
+
 /* GET /api/textbooks/:slug — single textbook detail */
 router.get('/:slug', (req, res) => {
   const t = db.prepare('SELECT * FROM textbooks WHERE slug=? AND is_active=1').get(req.params.slug);