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/learningService.js

@@ -154,6 +154,28 @@ export async function deleteCachedContent(topicId) {
   return db.deleteContent(topicId);
 }
 
+function slugify(label) {
+  const base = String(label || '')
+    .toLowerCase()
+    .normalize('NFKD')
+    .replace(/\p{Diacritic}/gu, '')
+    .replace(/[^a-z0-9]+/g, '-')
+    .replace(/^-+|-+$/g, '');
+  return base || 'topic';
+}
+
+async function pickUniqueTopicId(label) {
+  const existing = await db.getTopics();
+  const used = new Set(existing.map((t) => t.id));
+  const base = slugify(label);
+  if (!used.has(base)) return base;
+  for (let i = 2; i < 1000; i++) {
+    const candidate = `${base}-${i}`;
+    if (!used.has(candidate)) return candidate;
+  }
+  return `${base}-${Date.now().toString(36)}`;
+}
+
 export async function generateCustomTopic(label) {
   const result = await callLLM({
     task: 'topic.custom',
@@ -168,7 +190,12 @@ export async function generateCustomTopic(label) {
   const emitted = result.toolUses[0]?.input;
   if (!emitted) throw new Error('Could not process custom topic. Please try again.');
 
-  const newTopic = { ...emitted, id: 'custom_' + Date.now().toString(36) };
+  const id = await pickUniqueTopicId(emitted.label);
+  const newTopic = {
+    ...emitted,
+    id,
+    learning_relevance: emitted.learning_relevance || 'standard',
+  };
   await db.upsertTopic(newTopic);
   return newTopic;
 }