Feature Request: Bulk Upload Attendees via CSV for Event Invites & Check-ins

## Problem description

OCG currently requires attendees to have an account and sign up for an event before they can be added as attendees. This makes it difficult to manage attendees who are already members of a group or who attended an event but did not register through OCG.

There is currently no simple way to bulk upload attendee details from a CSV to mark their attendance/check-in or invite them to an event.

## Proposed feature

Add a **CSV bulk attendee upload** capability for events.

The feature should allow organizers to upload a CSV containing attendee details, primarily email addresses, to:

* Bulk invite people to an event before or during the event.
* Bulk add existing group members who attended the event.
* Mark attendees as checked-in/attended through a bulk upload.
* Reduce the need for attendees to individually register when organizers already have their details.

## Scope and behavior

* Organizers can upload a CSV containing attendee email addresses and relevant attendee details.
* OCG should match uploaded emails against existing accounts/group members where possible.
* Existing users can be added to the event directly and/or marked as checked in.
* For users without an OCG account:

* Send an invitation to join/sign up for the event.
* Their attendance should be considered confirmed only after they create/sign into an account and accept the invitation.
* The organizer should receive a summary of successfully processed, matched, invited, and failed records.
* The feature should support both **pre-event invitations** and **during/post-event bulk attendance management**.

## Motivation

Event organizers often already have attendee information from registrations, community groups, previous platforms, or on-site check-ins. Requiring every attendee to individually create an OCG account and register creates unnecessary friction and makes post-event attendance management cumbersome.

A bulk CSV upload would provide organizers with a practical way to **invite attendees, onboard existing community members, and efficiently record event attendance/check-ins at scale**.

Based on - https://cloud-native.slack.com/archives/C0AKJ12C4MD/p1785593581452209