feat: set up authentication infrastructure with NextAuth.js v5

- Install next-auth@beta, bcryptjs, zod, and @auth/prisma-adapter
- Create auth.ts with credentials provider and JWT session strategy
- Create auth.config.ts for edge-compatible middleware configuration
- Add NextAuth type extensions for session and JWT
- Create API route handler for authentication
- Configure environment variables for NextAuth
- Add AUTH_SETUP.md documentation

Task: 1. Set up authentication infrastructure
Requirements: 1.1, 2.1

types/next-auth.d.ts

@@ -0,0 +1,21 @@
+import { DefaultSession } from "next-auth"
+
+declare module "next-auth" {
+  interface Session {
+    user: {
+      id: string
+    } & DefaultSession["user"]
+  }
+
+  interface User {
+    id: string
+    email: string
+    name: string | null
+  }
+}
+
+declare module "next-auth/jwt" {
+  interface JWT {
+    id: string
+  }
+}