Add option to restrict anonymous access to order URLs (#4735)

* Add option to restrict anonymous access to order URLs

By default, users who place orders while logged in can still access
their order URLs without authentication. This raises potential
security risks, particularly if order confirmation emails are
forwarded.

This commit introduces an organiser-level setting to disable anonymous
access for such orders. When enabled, unauthenticated attempts to access
URLs starting with `/order/`, which are intended for the customer, are
redirected to the login page. Upon successful authentication, the user
is redirected back to the original order URL.

It is important to note that this change does not impact routes intended
for attendees (e.g., `/ticket/*`), which remain accessible without
authentication.

* Change name of setting for future clarity

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

* Update message wording

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

* Eliminate database query

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

* Rename feature flag to fix breaking tests

* Refactor order access verification code into `OrderDetailsMixin`

* Add test for logged-in customer accessing another customer's order

* Refactor order access conditions to remove nesting

* Handle case where customer is not yet verified

* Add additional information to help message

* Fix multidomain issue

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

* Merge order/position variants into single tests

* Add docstring explaining return type of `order` property

* Apply suggestion from @raphaelm

* Fix indentation

---------

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>
Co-authored-by: Raphael Michel <michel@rami.io>

src/pretix/api/serializers/organizer.py

@@ -443,6 +443,7 @@ class OrganizerSettingsSerializer(SettingsSerializer):
         'customer_accounts',
         'customer_accounts_native',
         'customer_accounts_link_by_email',
+        'customer_accounts_require_login_for_order_access',
         'invoice_regenerate_allowed',
         'contact_mail',
         'imprint_url',