feat: Add event co-managers permissions and allow managers/admins to signup guests on skill-restricted shifts

backend/apps/events/models.py

@@ -18,9 +18,19 @@ class Event(models.Model):
     end_date = models.DateField()
     is_active = models.BooleanField(default=True, help_text="Aktiv/Deaktiviert Status")
     created_by = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.SET_NULL, null=True, related_name='created_events')
+    co_managers = models.ManyToManyField(settings.AUTH_USER_MODEL, blank=True, related_name='managed_events', help_text="Weitere Verwalter dieser Veranstaltung")
     created_at = models.DateTimeField(auto_now_add=True)
     updated_at = models.DateTimeField(auto_now=True)
 
+    def is_user_manager_or_admin(self, user):
+        if not user or not user.is_authenticated:
+            return False
+        if user.is_admin_user or user.is_staff or user.is_superuser:
+            return True
+        if self.created_by_id == user.id:
+            return True
+        return self.co_managers.filter(id=user.id).exists()
+
     def __str__(self):
         return f"{self.title} ({'Aktiv' if self.is_active else 'Deaktiviert'})"