/** Human-readable error or `null` if valid. Consumed by `routes/sync.ts` at
 *  save-time so invalid crons are rejected with a 400 instead of silently
 *  being "enabled but never fires". */
export declare function validateCron(expr: string): string | null;
export declare function startSyncScheduler(): void;
export declare function stopSyncScheduler(): void;
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