Fix ESLint type import and add better-auth to serverExternalPackages

Prevents Turbopack from statically analyzing better-auth's kysely adapter,
which imports symbols removed in kysely 0.29. Also fixes @typescript-eslint
consistent-type-imports warning on the Db type alias.

https://claude.ai/code/session_014126WHVRT8mftmqkU6dajG

next.config.js

@@ -8,7 +8,7 @@ import "./src/env.js";
 const config = {
   output: "standalone",
   reactCompiler: true,
-  serverExternalPackages: ["pg"],
+  serverExternalPackages: ["pg", "better-auth"],
 };
 
 export default config;