feat: add script field to ScheduledTask type and database layer

Adds optional `script` field to the ScheduledTask interface, with a
migration for existing DBs and full support in createTask/updateTask.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/types.ts

@@ -58,6 +58,7 @@ export interface ScheduledTask {
   group_folder: string;
   chat_jid: string;
   prompt: string;
+  script?: string | null;
   schedule_type: 'cron' | 'interval' | 'once';
   schedule_value: string;
   context_mode: 'group' | 'isolated';