Email notifications for Groupfolders “advanced permissions” user owners when folder quota reaches thresholds

[Uri2001]

Is your feature request related to a problem? Please describe.
In Team folders (Groupfolders) we can assign an “owner” in Manage advanced permissions.
Currently, when storage/quota for a Team folder is close to being full (or becomes full), there is no way to automatically notify the responsible person by email.

In our setup, the “owner” field is used as a responsible user for a specific Team folder. Without an email notification, the folder owner often learns about the quota issue only after users start reporting upload errors.

Describe the solution you'd like
Please add an option to send quota threshold notifications (e.g. 80% / 90% / 95% / 100%) to the owner configured in “Manage advanced permissions”, when the owner is a user (not a group).

Expected behavior:

• If the Team folder has an “owner” set to a user account, send an email notification to that user when the folder reaches configured quota thresholds.
• If the “owner” is a group, either:
  • do nothing (simplest), or
  • optionally notify all members of that group / group admins (only if feasible and configurable).
• Notification should include at least:
  • Team folder name (and ideally its internal ID/path if useful)
  • current usage and quota (e.g. “9.2 TB / 10 TB (92%)”)
  • threshold that triggered the message
  • link to the Team folder settings page (if available)
    Nice-to-have:
• Per-folder configuration of thresholds / enable/disable.
• Avoid spamming: send once per threshold and only re-send after usage drops below threshold and reaches it again.

Describe alternatives you've considered

• External monitoring (Prometheus/Zabbix) that reads storage usage and sends emails — works, but requires extra integration and does not naturally map to the “owner” defined in Groupfolders.
• Relying on admin notifications only — in large instances, admins are not the best recipients; the folder owner is.

Additional context

• We use the “owner” in Manage advanced permissions as the primary responsibility/contact person for each Team folder.
• This feature would greatly reduce downtime and user-facing issues caused by full Team folder quotas.

[provokateurin]

IMO it only makes sense to notify instance admins, since they are the ones that can create and modify Team folders.
The underlying problem is, that the quota isn't shown in the frontend when the user navigates to a Team folder (see #44). During an upload they should get a proper "not enough space" error though.