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/components/admin/TestManager.jsx

@@ -34,7 +34,7 @@ const TestManager = () => {
     loadData();
   }, [selectedTopic]);
 
-  const handleGenerate = async (topic, count = 10) => {
+  const handleGenerate = async (topic, count = 5) => {
     setLoadingTopicId(topic.id);
     setError(null);
     try {
@@ -97,8 +97,8 @@ const TestManager = () => {
           {questions.length === 0 ? (
             <Card className="text-center py-12 text-fg-muted border-dashed border-2">
               <p>No questions generated for this topic yet.</p>
-              <Button className="mt-4" onClick={() => handleGenerate(selectedTopic, 10)} disabled={loadingTopicId === selectedTopic.id}>
-                Generate 10 Questions
+              <Button className="mt-4" onClick={() => handleGenerate(selectedTopic, 5)} disabled={loadingTopicId === selectedTopic.id}>
+                Generate 5 Questions
               </Button>
             </Card>
           ) : (