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

packages/backend/convex/betterAuth/auth.ts

@@ -0,0 +1,5 @@
+import { createAuth } from "../auth";
+import { getStaticAuth } from "@convex-dev/better-auth";
+
+// Export a static instance for Better Auth schema generation
+export const auth = getStaticAuth(createAuth);