test: add vitest baseline for critical payment and auth guards

package.json

@@ -7,7 +7,9 @@
     "dev:strict": "next dev",
     "build": "next build",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "@convex-dev/better-auth": "^0.11.3",
@@ -56,6 +58,7 @@
     "eslint": "^9",
     "eslint-config-next": "16.2.1",
     "tailwindcss": "^4",
-    "typescript": "^5"
+    "typescript": "^5",
+    "vitest": "^3.2.4"
   }
 }