feat: phase 4 of AI pipeline hardening — quiz & content quality

- src/lib/random.js: Fisher–Yates shuffle/sample/pickInt; replace every
  biased .sort(() => 0.5 - Math.random()) site in testService.
- testService: debias correctIndex via prompt + runtime re-roll (up to 2x
  when one position holds >50%); quality gate rejecting <4 distinct
  options, banned filler ("all of the above" etc) and explanations
  shorter than 20 chars; dedup new questions against the existing bank
  via normalised question text.
- Quiz schema/tool/prompt require difficulty ('easy'|'medium'|'hard');
  db.getQuizBank defaults legacy records to 'medium' on read.
- learningService.generateCustomTopic: kebab-case slug ID from the
  polished label with collision suffixes; default learning_relevance
  'standard' when the model omits it.
- Tests for random helpers, dedup/quality-gate behaviour and the
  extended quiz schema.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/db.js

@@ -90,10 +90,15 @@ export async function deleteContent(topicId) {
 
 // ── Quiz Banks ───────────────────────────────────────────────────────────────
 
+function normalizeQuizQuestion(q) {
+  if (!q || typeof q !== 'object') return q;
+  return q.difficulty ? q : { ...q, difficulty: 'medium' };
+}
+
 export async function getQuizBank(topicId) {
   try {
     const r = await pb.collection('quiz_banks').getFirstListItem(`topic_id="${topicId}"`);
-    return r.questions || [];
+    return (r.questions || []).map(normalizeQuizQuestion);
   } catch { return []; }
 }