Support re-joining the server

From a [post-conference support request](https://discord.com/channels/1253767144265482280/1262710639852261508/1262710639852261508) at EP2024:
A user left the server after the conference, but wanted to re-join to access some shared URLs.

#### Requirement
Users shall be able to re-join the server with the same ticket details and the same Discord account.

#### Suggested implementation
We currently store a list of all tickets used for registration to prevent multiple users from registering with the same ticket.

Let's add the Discord account ID to these records, and only prevent re-registrations with different account IDs.