Implement Account Sweeping Service & Cron Job

[Olowodarey]

A key feature of Nestera is sweeping excess funds from user wallets into designated savings products. We need a background service that evaluates accounts periodically and executes the sweep logically.

Acceptance Criteria

• Install @nestjs/schedule and configure ScheduleModule globally.
• Create a SweepTasksService.
• Implement a @Cron() job that runs periodically to fetch users with active auto-sweep settings.
• Calculate excess funds mathematically based on user threshold preferences.
• Stub out the actual transfer logic (which might engage the StellarService).

Useful Resources

• NestJS Task Scheduling