feat(bisaya-course): add new lesson and update course creation logic

- Introduced a new lesson titled "Zahlen & Preise" to enhance the numerical curriculum in Bisaya.
- Updated the course creation logic to check for existing "Familien"-Bisaya courses, ensuring idempotency in course creation.
- Enhanced the lesson didactics by mapping legacy titles to current lesson keys, improving data consistency.
- Adjusted the course generation script to limit Bisaya courses to German-speaking learners only, streamlining course offerings.

backend/scripts/create-bisaya-course-content.js

@@ -5169,6 +5169,10 @@ async function findOrCreateSystemUser() {
 
 function getExercisesForLesson(lesson) {
   const lessonTitle = lesson.title;
+  // Alte Kurstitel (DB noch nicht migriert)
+  if (lessonTitle === 'Zahlen & Preise' && BISAYA_EXERCISES['Zahlen 1–20']) {
+    return BISAYA_EXERCISES['Zahlen 1–20'];
+  }
   // Suche nach exaktem Titel
   if (BISAYA_EXERCISES[lessonTitle]) {
     return BISAYA_EXERCISES[lessonTitle];
@@ -5254,6 +5258,7 @@ async function createBisayaCourseContent() {
         'Zeitformen - Grundlagen',
         'Zeit & Datum',
         'Einkaufen & Preise',
+        'Zahlen & Preise',
         'Zahlen 1–20',
         'Zahlen: Zehner',
         'Zahlen: Hunderter',