fix: replace 'as any' with proper Db type annotation

Replaced the type assertion 'as any' with a proper 'as Db' type
annotation for the MongoDB adapter in lib/auth.ts. This addresses
the code review feedback to ensure type safety.

Changes:
- Import Db type from mongodb package
- Use 'as Db' instead of 'as any' for mongoClient.db() call

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: snomiao

lib/auth.ts

@@ -1,6 +1,6 @@
 import { betterAuth } from "better-auth";
 import { mongodbAdapter } from "better-auth/adapters/mongodb";
-import { MongoClient } from "mongodb";
+import { MongoClient, type Db } from "mongodb";
 
 // Backward compatibility with NextAuth environment variables
 const getAuthConfig = () => {
@@ -36,7 +36,7 @@ const MONGODB_URI = process.env.MONGODB_URI ?? "mongodb://localhost:27017";
 const mongoClient = new MongoClient(MONGODB_URI);
 
 export const auth = betterAuth({
-  database: mongodbAdapter(mongoClient.db() as any),
+  database: mongodbAdapter(mongoClient.db() as Db),
   baseURL: config.baseURL,
   emailAndPassword: {
     enabled: false,