Delete empty Talk threads

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Feature request

Which Nextcloud Version are you currently using: (see administration page)
Nextcloud Hub 25 Autumn (32.0.3)

Is your feature request related to a problem? Please describe.
Yes. Currently it is not possible to delete a Talk thread if the message that originally created the thread has been deleted.

In our case, a user posted a message which automatically created a thread. This original message was later deleted. After that, the thread itself still exists, but it is completely empty (no messages) and cannot be deleted via the UI.

This results in “ghost threads” that have no content, no purpose, and clutter the conversation view.

Describe the solution you'd like
Empty threads (especially those whose root/original message was deleted) should either:

• be deleted automatically when the original message is removed, or
• be manually deletable by users (or moderators) via the UI.

This would prevent orphaned, empty threads from remaining permanently.

Describe alternatives you've considered
We looked for administrative or database-level cleanup options, but this is not a viable or safe solution for regular operation. From a user perspective, there is currently no workaround to remove such empty threads.

Additional context
This issue can easily occur in normal usage when users delete messages that have already started a thread. Over time, this leads to multiple empty threads that cannot be interacted with or removed.

No files, tests, or code entry points are named. Start by reproducing the empty-thread case in Nextcloud Talk after deleting a thread's original message, then trace the existing thread and message deletion flows. Done means orphaned empty threads can be removed safely, either automatically or through the UI, with the applicable user or moderator permissions defined.