feat(backend): add admin plugin and create-admin script (#80)

* feat(backend): add admin plugin and create-admin script

Add Better Auth admin plugin for role-based user management.
Includes a CLI script to create admin accounts.

Co-authored-by: Ona <no-reply@ona.com>

* fix(backend): guard against missing BETTER_AUTH_SECRET

Co-authored-by: Ona <no-reply@ona.com>

---------

Co-authored-by: Ona <no-reply@ona.com>

apps/aelis-backend/src/auth/index.ts

@@ -1,10 +1,16 @@
 import { betterAuth } from "better-auth"
 import { drizzleAdapter } from "better-auth/adapters/drizzle"
+import { admin } from "better-auth/plugins"
 
 import type { Database } from "../db/index.ts"
+
 import * as schema from "../db/schema.ts"
 
 export function createAuth(db: Database) {
+	if (!process.env.BETTER_AUTH_SECRET) {
+		throw new Error("BETTER_AUTH_SECRET is not set")
+	}
+
 	return betterAuth({
 		database: drizzleAdapter(db, {
 			provider: "pg",
@@ -13,6 +19,7 @@ export function createAuth(db: Database) {
 		emailAndPassword: {
 			enabled: true,
 		},
+		plugins: [admin()],
 	})
 }