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

auth.config.ts

@@ -0,0 +1,28 @@
+import type { NextAuthConfig } from "next-auth"
+
+export const authConfig = {
+  pages: {
+    signIn: "/auth/login",
+    signOut: "/auth/logout",
+    error: "/auth/error",
+  },
+  callbacks: {
+    authorized({ auth, request: { nextUrl } }) {
+      const isLoggedIn = !!auth?.user
+      const isOnAuthPage = nextUrl.pathname.startsWith("/auth")
+      
+      // Allow access to auth pages when not logged in
+      if (isOnAuthPage) {
+        return true
+      }
+      
+      // Redirect to login if not authenticated
+      if (!isLoggedIn) {
+        return false
+      }
+      
+      return true
+    },
+  },
+  providers: [], // Providers will be added in auth.ts
+} satisfies NextAuthConfig