Add an auth:migrate npm script for Better Auth's own migrations

Installs @better-auth/cli as a devDependency instead of fetching it via
npx each time, and updates the README to use the new script.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -50,7 +50,7 @@ etc). A `docker-compose.yml` at the repo root starts one:
 docker compose up -d           # starts Postgres on localhost:5433
 cp .env.example .env.local     # sets DATABASE_URL to match
 npm run db:migrate             # applies this project's Drizzle migrations
-npx @better-auth/cli migrate   # applies Better Auth's own tables
+npm run auth:migrate           # applies Better Auth's own tables
 npm run db:seed                # seeds a sample Brand, menu, and Machines
 npm run dev
 ```
@@ -72,7 +72,7 @@ Generate and set the `BETTER_AUTH_SECRET` environment variable in your
 `.env.local`:
 
 ```bash
-npx -y @better-auth/cli secret
+npx @better-auth/cli secret
 ```
 
 See the [Better Auth documentation](https://www.better-auth.com) for more on