Add better-auth integration and update dependencies in package.json and bun.lock

packages/backend/convex/convex.config.ts

@@ -0,0 +1,7 @@
+import { defineApp } from "convex/server";
+import betterAuth from "./betterAuth/convex.config";
+
+const app = defineApp();
+app.use(betterAuth);
+
+export default app;