fix: superadmin course access + navigate to correct course from courses page

Backend: list_students, add_student, import_students now bypass
tutor_courses check for superadmins, matching the existing pattern
in list_assigned_tutors. A superadmin creating a new course was
getting 401 when accessing it because no tutor_courses row exists
for them.

Frontend: courses page passes ?courseId= to students/sessions links;
both pages now pre-select the matching course on mount instead of
always defaulting to courses[0].

frontend/src/routes/admin/courses/+page.svelte

@@ -139,8 +139,8 @@
               {/if}
               <td style="padding:12px 14px;text-align:right">
                 <div style="display:flex;gap:6px;justify-content:flex-end">
-                  <a href="/admin/students" class="btn ghost sm">Studierende</a>
-                  <a href="/admin/sessions" class="btn ghost sm">Sitzungen</a>
+                  <a href="/admin/students?courseId={course.id}" class="btn ghost sm">Studierende</a>
+                  <a href="/admin/sessions?courseId={course.id}" class="btn ghost sm">Sitzungen</a>
                 </div>
               </td>
             </tr>

frontend/src/routes/admin/sessions/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
   import { onMount } from 'svelte';
+  import { page } from '$app/state';
   import { api } from '$lib/api';
   import type { Course, Room, Session } from '$lib/types';
   import Icon from '$lib/components/Icon.svelte';
@@ -23,8 +24,9 @@
   onMount(async () => {
     courses = await api.admin.courses.list();
     rooms = await api.admin.rooms.list();
-    const first = courses[0];
-    if (first) selectedCourseId = first.id;
+    const paramId = page.url.searchParams.get('courseId');
+    const match = paramId ? courses.find(c => c.id === Number(paramId)) : null;
+    selectedCourseId = (match ?? courses[0])?.id ?? null;
   });
 
   $effect(() => {

frontend/src/routes/admin/students/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
   import { onMount } from 'svelte';
+  import { page } from '$app/state';
   import { api } from '$lib/api';
   import type { Course, Student } from '$lib/types';
   import Icon from '$lib/components/Icon.svelte';
@@ -13,8 +14,9 @@
 
   onMount(async () => {
     courses = await api.admin.courses.list();
-    const first = courses[0];
-    if (first) selectedCourseId = first.id;
+    const paramId = page.url.searchParams.get('courseId');
+    const match = paramId ? courses.find(c => c.id === Number(paramId)) : null;
+    selectedCourseId = (match ?? courses[0])?.id ?? null;
   });
 
   $effect(() => {