feat(settings): expose federatedRegistrationOpen in /settings/instance + /instance/info

Surfaces the federatedRegistrationOpen flag (Task 1 schema column) on the
admin settings GET/PATCH endpoints and the public /api/instance/info
endpoint. Closes the 3 deferred TypeScript errors from Task 2 by
populating the now-required InstanceAdminSettings/InstanceInfoResponse
field.

Adds smoke tests (routes/instance.test.ts, routes/settings.test.ts) that
lock in the JSON contract the Connections UI (Task 21) and admin
RegistrationPanel (Task 15) consume, plus boolean-validation coverage
for the PATCH path. Updates docs/systems/admin.md with the new field in
both InstanceAdminSettings and the public info response schema.

packages/server/src/routes/instance.ts

@@ -22,6 +22,7 @@ export async function instanceRoutes(app: FastifyInstance): Promise<void> {
       name: instanceName,
       version: BACKSPACE_VERSION,
       registrationOpen,
+      federatedRegistrationOpen: settings?.federatedRegistrationOpen === 1,
     };
 
     return reply.code(200).send(response);