feat: Query attendees for "Thank you for Attending" or "Sorry you couldn't make it" message types. 🤖

[mrjbj]

What changes I've made

Add two new recipient groups to the organizer messaging system: "Attendees who checked in" and "Attendees who did not check in", scoped to a specific check-in list. This allows follow-on "Thank you for attending" or "Sorry you couldn't make it" messaging from the organizer, scoped to allow different messages per check-in list.

Backend:

• Added CHECKED_IN_ATTENDEES and NOT_CHECKED_IN_ATTENDEES to MessageTypeEnum
• Added repository methods on AttendeeRepository using whereHas('check_ins') / whereDoesntHave('check_ins'), scoped by check-in list and filtered to attendees whose ticket type is on that check-in list
• Added check_in_list_id to SendMessageDTO, request validation, and send pipeline
• Added recipient count estimation for both new types in SendMessageHandler
• 3 unit tests covering the new send paths

Frontend:

• Added two options to the "Select attendee group" dropdown in Send Message modal
• Added a check-in list selector (single select) that appears when either check-in type is chosen
• Added type labels for the message list display
• Translations for all 18 supported locales

Why I've made these changes

Event organizers commonly want to send post-event follow-up emails — "thank you for coming" to attendees who checked in, or "sorry we missed you" to those who didn't show. This was previously impossible without manually tracking attendance outside the platform.

The check-in list selector ensures correct scoping for events with multiple check-in lists (e.g., "Main Entry" + "VIP Dinner") — an attendee who checked into the main event but missed the VIP dinner appears in the right group for each list.

How I've tested these changes

• Tested in production deployment with real event data and check-in lists
• Verified correct attendee filtering: only attendees whose ticket type is on the selected check-in list are included
• Verified attendees without a check-in list on their ticket type are excluded from both groups
• Verified message delivery and recipient status tracking
• npx tsc --noEmit — no new TypeScript errors
• Unit tests pass
• Works with suppression system so suppressed recipients do not get further email.

Checklist

• I have read the contributing guidelines.
• My code follows the coding standards of the project.
• I have tested my changes, and they work as expected.
• I understand that this PR will be closed if I do not follow the contributor
  guidelines and if this PR template is left unedited.