feat(admin): add registration policy settings and disabled register state

apps/admin/src/lib/auth/server.ts

@@ -1,5 +1,5 @@
 import { normalizeRole, type Role } from "@cms/content/rbac"
-import { db } from "@cms/db"
+import { db, isAdminSelfRegistrationEnabled } from "@cms/db"
 import { betterAuth } from "better-auth"
 import { prismaAdapter } from "better-auth/adapters/prisma"
 import { toNextJsHandler } from "better-auth/next-js"
@@ -43,8 +43,7 @@ export async function isInitialOwnerRegistrationOpen(): Promise<boolean> {
 }
 
 export async function isSelfRegistrationEnabled(): Promise<boolean> {
-  // Temporary fallback until registration policy is managed from admin settings.
-  return process.env.CMS_ADMIN_SELF_REGISTRATION_ENABLED === "true"
+  return isAdminSelfRegistrationEnabled()
 }
 
 export async function canUserSelfRegister(): Promise<boolean> {