feat: reduce initial question batch size for a topic to 5

When a topic's quiz bank is empty (or below the requested count), we
previously seeded it with a fresh batch of 10 questions. That meant the
first weekly quiz for any new topic triggered a 10-question LLM call —
heavy for what's ultimately a 1-question sample for review topics, and
overkill for the typical 5-question primary topic.

- forceGenerateTopicQuestions default count: 10 → 5
- getOrGenerateTopicQuestions seed amount: 10 → 5
- TestManager "Generate" defaults + empty-state button copy: 10 → 5
- QUIZ_SYSTEM difficulty hint: rewritten for a 5-question batch (2 easy
  / 2 medium / 1 hard) with explicit "scale proportionally for larger
  batches" so admins can still generate 10+ via TestManager when they
  want more depth.

Tests 61/61 pass, lint clean (0 errors), build clean.

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

src/lib/testService.js

@@ -7,7 +7,7 @@ const QUIZ_SYSTEM = `You are a quiz generator for Respellion, an internal IT com
 You generate multiple-choice questions to test employee knowledge on specific topics.
 Always write in clear, professional English.
 
-Emit questions through the emit_quiz_questions tool. Each question has exactly four options; correctIndex is 0-based; mix difficulty roughly 4 easy / 4 medium / 2 hard.`;
+Emit questions through the emit_quiz_questions tool. Each question has exactly four options; correctIndex is 0-based. For a typical 5-question batch, mix difficulty roughly 2 easy / 2 medium / 1 hard; scale the ratio proportionally for larger batches.`;
 
 const cachedSystem = (text) => [{ type: 'text', text, cache_control: { type: 'ephemeral' } }];
 
@@ -67,7 +67,7 @@ export async function getCachedQuiz(userId, weekNumber) {
   return db.getCachedQuiz(userId, weekNumber);
 }
 
-export async function forceGenerateTopicQuestions(topic, count = 10) {
+export async function forceGenerateTopicQuestions(topic, count = 5) {
   let bank = await db.getQuizBank(topic.id);
 
   const prompt = `Generate exactly ${count} multiple-choice quiz questions for this knowledge topic and emit them via the emit_quiz_questions tool:
@@ -105,7 +105,9 @@ async function getOrGenerateTopicQuestions(topic, count) {
   let bank = await db.getQuizBank(topic.id);
 
   if (bank.length < count) {
-    await forceGenerateTopicQuestions(topic, 10);
+    // Seed an empty/low bank with a small initial batch; admins can grow it
+    // explicitly via TestManager when they want more depth per topic.
+    await forceGenerateTopicQuestions(topic, 5);
     bank = await db.getQuizBank(topic.id);
   }