[BUG] Allow only one guest participant via email link

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Steps to reproduce

1. Host a Talk "Webinar"
2. Invite guest via email invite (a csv with one email)
3. Open same link on two different private tabs

Expected behaviour

Nextcloud Talk should not allow the latter to join

Actual behaviour

Both people using the same link can join, while Talk reports only one participant

Talk app

Talk app version: 24.0.1

Custom Signaling server configured: 2.1.1~docker (Nextcloud AIO)
Custom TURN server configured: yes (Nextcloud AIO)

Custom STUN server configured: yes (Nextcloud AIO)

Browser

Microphone available: yes

Camera available: yes

Operating system: Any

Browser name: Any

Browser version: N/A

Browser log

N/A

Start by reproducing the issue in Nextcloud Talk 24.0.1 using the same email invite link in two private tabs, then trace the guest-link join flow and its interaction with the configured signaling server. Done means the second guest cannot join through the same link and Talk reports the correct participant count; add or update regression coverage where the join behavior is tested.