Stundenplan Phase 3b: 3 more Stammdaten managers, first constraint editor, full test coverage

Frontend additions in studio-v2:
  - LehrerManager / FaecherManager / RaeumeManager — same CRUD pattern as
    Klassen, with entity-specific form fields and table columns.
  - regeln/TeacherUnavailableDayEditor — first constraint editor, joins
    against teachersApi to render a readable name in the dropdown and
    list. Falls back to a guidance banner when no teachers exist yet.
  - page.tsx wires up the new tabs; data-testid attributes added across
    managers so the Playwright suite can target them deterministically.

Tests:
  - school-service: timetable_constraints_more_test.go fills the
    remaining 9 constraint DTOs (TeacherMaxHoursDay/Week,
    TeacherExcludedSubject/Room, SubjectMinDayGap,
    SubjectContiguousWhenRepeated, SubjectDoubleLesson, ClassNoGaps,
    RoomRequiresType). 66 subtests total, all green.
  - studio-v2: e2e/stundenplan.spec.ts covers the page shell, tab
    navigation, Klassen CRUD with mocked backend, constraint editor's
    empty-teacher fallback, sidebar entry. All school-service calls
    intercepted via page.route() so the suite is hermetic.

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

studio-v2/app/stundenplan/_components/KlassenManager.tsx

@@ -68,7 +68,7 @@ export function KlassenManager() {
     : 'bg-white border-slate-300 text-slate-900 placeholder-slate-400'
 
   return (
-    <div className="space-y-4">
+    <div className="space-y-4" data-testid="klassen-manager">
       <div className="flex items-center justify-between">
         <h2 className={`text-xl font-semibold ${isDark ? 'text-white' : 'text-slate-900'}`}>
           Klassen ({classes.length})