Add Task Scheduler API on top of BackgroundJobs (cron-like fluent scheduling)

[antosubash]

Background

BackgroundJobs already persists ScheduledAt and CronExpression columns, but there is no fluent API to register recurring jobs at module-startup time, nor a worker that wakes up and enqueues due jobs. Laravel's Kernel::schedule() is the canonical example: every cron tick the scheduler enumerates definitions and dispatches due ones.

Motivation

• Most apps need recurring work: nightly cleanup, daily digest emails, hourly sync, etc.
• Schema is already in place; only the dispatcher + DSL are missing
• Eliminates per-app cron + AppHost glue

Design sketch

public interface IScheduler
{
    IScheduledJob<T> Job<T>() where T : IBackgroundJob;
}

public interface IScheduledJob<T>
{
    IScheduledJob<T> Cron(string expression);
    IScheduledJob<T> Daily();
    IScheduledJob<T> DailyAt(string time);          // "03:00"
    IScheduledJob<T> EveryMinutes(int minutes);
    IScheduledJob<T> Hourly();
    IScheduledJob<T> Weekdays();
    IScheduledJob<T> Timezone(string tz);
    IScheduledJob<T> WithoutOverlapping();          // mutex via DB
    IScheduledJob<T> OnOneServer();                 // leader election
    IScheduledJob<T> WithPayload(object payload);
}

Module registers schedules in IModule.ConfigureServices:

scheduler.Job<NightlyAuditPurge>().DailyAt("02:00").Timezone("UTC");
scheduler.Job<DigestEmail>().Cron("0 8 * * MON-FRI").WithoutOverlapping();

• A hosted SchedulerService polls every 30s, computes due jobs from registered definitions, inserts into BackgroundJobs queue
• Distributed safety: WithoutOverlapping writes a mutex row keyed by job-name; OnOneServer uses an instance-id leader lease (DB row with TTL)
• Cron parsing via Cronos (already on .NET; widely used)

Acceptance criteria

• IScheduler exposed from BackgroundJobs.Contracts
• SchedulerService runs as IHostedService in worker process
• Mutex (WithoutOverlapping) and leader (OnOneServer) primitives implemented and tested
• Failure isolation: one bad definition does not stop the loop
• xUnit tests cover cron parsing, due-time calc, mutex contention
• Docs: how to register a scheduled job
• CLI: sm jobs list-scheduled prints next-run-at

References

• Laravel scheduling: https://laravel.com/docs/scheduling
• Cronos: https://github.com/HangfireIO/Cronos