feat: add dedicated sign-up config for oauth (#13610)

Currently, it is only possible to disable all registrations. However, it
would be helpful if you could disable normal registration but enable
OAuth registration.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added a setting to enable/disable new user signups via OAuth (default:
enabled).
* Admin Settings (Authentication) now includes a toggle for OAuth
signups.
* OAuth signup flow now respects this setting, preventing new
registrations via OAuth when disabled.
  * Self-hosted configuration schema updated to include the new option.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Hudint Finn Weigand <dev@hudint.de>
Co-authored-by: DarkSky <darksky2048@gmail.com>
Co-authored-by: DarkSky <25152247+darkskygit@users.noreply.github.com>

.docker/selfhost/schema.json

@@ -148,6 +148,11 @@
           "description": "Whether allow new registrations.\n@default true",
           "default": true
         },
+        "allowSignupForOauth": {
+          "type": "boolean",
+          "description": "Whether allow new registrations via configured oauth.\n@default true",
+          "default": true
+        },
         "requireEmailDomainVerification": {
           "type": "boolean",
           "description": "Whether require email domain record verification before accessing restricted resources.\n@default false",