fix: make CRON_SECRET optional in environment configuration

Author: kingRayhan

src/env.ts

@@ -19,7 +19,7 @@ export const env = createEnv({
     S3_ACCESS_SECRET: z.string().min(1),
     S3_BUCKET: z.string().min(1),
 
-    CRON_SECRET: process.env.CRON_SECRET,
+    CRON_SECRET: z.string().optional(),
   },
   client: {
     NEXT_PUBLIC_MEILISEARCH_API_HOST: z.string().url(),