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>

package.json

@@ -16,6 +16,7 @@
     "db:pull": "drizzle-kit pull",
     "db:studio": "drizzle-kit studio",
     "db:seed": "tsx src/db/seed.ts",
+    "auth:migrate": "better-auth migrate",
     "lint": "eslint",
     "format": "prettier --write . && eslint --fix",
     "check": "prettier --check .",
@@ -38,6 +39,7 @@
     "zod": "^4.5.4"
   },
   "devDependencies": {
+    "@better-auth/cli": "^1.4.21",
     "@tanstack/devtools-vite": "latest",
     "@tanstack/eslint-config": "latest",
     "@tanstack/router-cli": "^1.132.0",