Feature Request: Schedule event emails

## Problem description

As event organizers, we often need to send announcements before an event (reminders, venue updates, agenda changes, preparation instructions, etc.). Currently, ocgroups only provides a "Send Email" option that sends the message immediately and only supports plain text.

This creates limitations for Oragniser:
- Emails cannot be scheduled to be sent at a future date and time.

## Proposed feature
1. Enhance the event email functionality by adding:
- Allow organizers to compose an email and schedule it to be sent at a specific date and time.
- Display scheduled emails in a list with their status (Scheduled, Sent, Cancelled, Failed).
- Allow editing or cancelling a scheduled email before it is sent.

## Scope and behavior
### Where this appears
Within the **Event** → **Email Attendees** section.

**Scheduling**
- Choose a future date and time.
- Use the organizer's timezone (or event timezone if already defined).
- Prevent scheduling in the past.
- Allow immediate sending as the current default behavior.

## Motivation
Many organizers send reminder emails a few days or hours before an event. Today this requires remembering to manually send them at the appropriate time. Scheduling emails would:

- Reduce manual work for organizers.
- Ensure reminders are delivered at the intended time.
- Help support organizers across different time zones.

These improvements would significantly enhance the organizer experience without changing the existing workflow for immediate emails.

## Optional

A option to schedule/send email basis of "People in Chapter", "People who are going to event x", "People who aren't going to event X". This feature was available in Bevy and helped us engaging people through target email for activities/challenege and promote the event audience growth.