Add Better Auth admin authentication

convex/betterAuth/adapter.ts

@@ -0,0 +1,17 @@
+import { createApi } from "@convex-dev/better-auth";
+
+import { createAuthOptions } from "./auth.js";
+import schema from "./schema.js";
+
+const createSchemaAuthOptions = (ctx: Parameters<typeof createAuthOptions>[0]) =>
+  createAuthOptions(ctx, { allowMissingSecret: true });
+
+export const {
+  create,
+  findOne,
+  findMany,
+  updateOne,
+  updateMany,
+  deleteOne,
+  deleteMany,
+} = createApi(schema, createSchemaAuthOptions);