feat(exam-prep F5): тренажёр случайных задач + /practice/next API (random|unsolved)

backend/src/routes/exam-prep.js

@@ -69,6 +69,38 @@ const SQL = {
      ORDER BY task_idx
   `),
 
+  /* Practice batch — RANDOM strategy.
+     Excludes long tasks (no auto-check) by default to keep UX consistent. */
+  practiceRandom: db.prepare(`
+    SELECT
+      id, task_idx, variant, task_type, text_html, figure_html, opts_json,
+      answer, solution_html, topic
+      FROM exam_tasks
+     WHERE exam_key = ?
+       AND task_type IN ('mc','open')
+     ORDER BY RANDOM()
+     LIMIT ?
+  `),
+
+  /* Practice batch — UNSOLVED strategy.
+     Excludes tasks the user has already solved correctly at least once. */
+  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
+      FROM exam_tasks t
+     WHERE t.exam_key = ?
+       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 ?
+  `),
+
   /* For attempt save: confirm the task belongs to a known track + user */
   getTaskExamKey: db.prepare(`SELECT exam_key FROM exam_tasks WHERE id = ?`),
 
@@ -168,6 +200,51 @@ router.get('/:examKey/variants/:n/tasks', (req, res) => {
 
 function safeJson(s) { try { return JSON.parse(s); } catch { return null; } }
 
+function shapeTask(r) {
+  return {
+    id:       r.id,
+    idx:      r.task_idx,
+    variant:  r.variant ?? null,
+    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 ?? null,
+  };
+}
+
+/* ── GET /api/exam-prep/:examKey/practice/next ──
+   Returns up to ?count=N tasks for the practice trainer.
+   ?strategy=random   — any mc/open task, fresh random sample
+            =unsolved — only tasks the user has not yet solved correctly
+   Long tasks are excluded (no auto-check available). */
+// @public-by-design: router-level authMiddleware (line 6) covers this route
+router.get('/:examKey/practice/next', (req, res) => {
+  const { examKey } = req.params;
+  if (!SQL.getTrack.get(examKey)) return res.status(404).json({ error: 'Unknown exam track' });
+
+  let count = Number(req.query.count) || 10;
+  count = Math.max(1, Math.min(count, 30));
+
+  const strategy = req.query.strategy === 'unsolved' ? 'unsolved' : 'random';
+  let rows;
+  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);
+  }
+
+  res.json({
+    strategy,
+    session_id: Date.now(),                  // ephemeral grouping key for these N attempts
+    tasks: rows.map(shapeTask),
+  });
+});
+
 /* ── POST /api/exam-prep/attempts ──
    Save a single attempt. Used by the variants view (F2) for "solution viewed"
    markers, and by the practice / mock views (F3+) for actual answer checking.