[Feature]: Deferred / Scheduled Command Execution

[eneshoxha]

Title: Add deferred and scheduled command execution support

Labels: enhancement, mediator, new-package

Body:

Problem

Some commands need to execute in the future (e.g., send a reminder email in 24 hours, retry a failed payment in 5 minutes). Currently, consumers must implement their own scheduling infrastructure.

Proposed Solution

Create a Cortex.Mediator.Scheduling package:

• ICommandScheduler interface:

public interface ICommandScheduler
{
    Task<string> ScheduleAsync<TCommand>(TCommand command, TimeSpan delay, CancellationToken ct = default)
        where TCommand : ICommand;
    Task<string> ScheduleAsync<TCommand>(TCommand command, DateTimeOffset executeAt, CancellationToken ct = default)
        where TCommand : ICommand;
    Task CancelAsync(string scheduleId, CancellationToken ct = default);
}

• In-memory implementation for development/testing
• Persistent implementation using a database table
• ScheduledCommandProcessor -- IHostedService that polls and dispatches due commands
• Optional integration hooks for Hangfire / Quartz.NET