Function update_settings 

pub async fn update_settings(
    pool: &PgPool,
    actor: &Actor,
    upd: SettingsUpdate,
) -> AppResult<Settings>

Saves the actor’s own notification settings (channels, Mattermost name, muted kinds).

Invariants

• Permission: any signed-in actor; only the actor’s own row changes. An invalid Mattermost username or a kind that cannot be muted is refused (400).
• Locks: none.
• Audit: user.notifications.
• Notifications / webhooks: none.
• Atomic: one transaction.