Email notification to the Send owner when a Send gets downloaded

Problem When sharing a file or text via Send, the owner currently has no way of knowing whether the recipient has actually retrieved it. The accessCount field exists, but you have to open the vault and check it manually. Especially for Sends with a short deletion date, it would be useful to know "the recipient got it, I can stop worrying" — similar to the download notifications offered by common file transfer services. Proposal A new Vaultwarden-specific global setting SEND_DOWNLOAD_NOTIFY (default: off) which, when enabled, notifies the owner of a Send via email when a recipient downloads it (file Sends, triggered when the download URL gets requested) or views it (text Sends): • off: no notification (default, current behavior) • always: notify on every download/view, including the current access count • first: only notify on the first download/view of each Send The notification only contains metadata the server already knows (date, IP address, send ty…