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/llmSchemas.js

@@ -122,6 +122,8 @@ export const learningAllSchema = z.object({
   infographic: infographicBodySchema,
 });
 
+const quizDifficultyEnum = z.enum(['easy', 'medium', 'hard']);
+
 const quizQuestionSchema = z.object({
   id: z.string().min(1),
   question: z.string().min(1),
@@ -129,6 +131,7 @@ const quizQuestionSchema = z.object({
   options: z.array(z.string().min(1)).length(4),
   correctIndex: z.number().int().min(0).max(3),
   explanation: z.string().min(1),
+  difficulty: quizDifficultyEnum,
 });
 
 export const quizQuestionsSchema = z.object({