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> [!IMPORTANT]
> Add functionality to send sign-in invitation emails, including backend
API, frontend UI, and email template support.
>
> - **Backend**:
> - Add `SIGN_IN_INVITATION` to `EmailTemplateType` in `schema.prisma`
and `migration.sql`.
> - New API route `send-sign-in-invitation/route.tsx` to handle sending
sign-in invitation emails.
> - Implement `sendSignInInvitationEmail()` in `admin-interface.ts`.
> - **Frontend**:
> - Update `page-client.tsx` to include sign-in invitation template
management.
> - Add `SendSignInInvitationDialog` in `page-client.tsx` for sending
invitations.
> - Modify `user-table.tsx` to include sign-in invitation actions.
> - **Email Templates**:
> - Add `sign-in-invitation.tsx` template.
> - Update `utils.tsx` to include metadata for sign-in invitation
emails.
>
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---------
Co-authored-by: Konsti Wohlwend <n2d4xc@gmail.com>
