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/__tests__/llmSchemas.test.js

@@ -108,7 +108,7 @@ describe('learning schemas', () => {
 });
 
 describe('quizQuestionsSchema', () => {
-  it('accepts a quiz with four options and a valid correctIndex', () => {
+  it('accepts a quiz with four options, a valid correctIndex and a difficulty', () => {
     const parsed = quizQuestionsSchema.parse({
       questions: [
         {
@@ -118,10 +118,12 @@ describe('quizQuestionsSchema', () => {
           options: ['A', 'B', 'C', 'D'],
           correctIndex: 2,
           explanation: 'C describes the buddy system best.',
+          difficulty: 'easy',
         },
       ],
     });
     expect(parsed.questions[0].options).toHaveLength(4);
+    expect(parsed.questions[0].difficulty).toBe('easy');
   });
 
   it('rejects three options or an out-of-range correctIndex', () => {
@@ -135,6 +137,7 @@ describe('quizQuestionsSchema', () => {
             options: ['A', 'B', 'C'],
             correctIndex: 0,
             explanation: 'e',
+            difficulty: 'medium',
           },
         ],
       }),
@@ -149,11 +152,27 @@ describe('quizQuestionsSchema', () => {
             options: ['A', 'B', 'C', 'D'],
             correctIndex: 4,
             explanation: 'e',
+            difficulty: 'medium',
           },
         ],
       }),
     ).toThrow();
   });
+
+  it('rejects a missing or unknown difficulty', () => {
+    const base = {
+      id: 'q',
+      question: 'q',
+      topicLabel: 't',
+      options: ['A', 'B', 'C', 'D'],
+      correctIndex: 0,
+      explanation: 'because',
+    };
+    expect(() => quizQuestionsSchema.parse({ questions: [base] })).toThrow();
+    expect(() =>
+      quizQuestionsSchema.parse({ questions: [{ ...base, difficulty: 'trivial' }] }),
+    ).toThrow();
+  });
 });
 
 describe('customTopicSchema', () => {