[Feature Request] Implement support for sending push notifications

[RequestPrivacy]

It would be really interesting to allow Sparrow to send push notifications for certain events (Transaction seen in mempool, transaction included in a block, new block etc.).

After the US government decided to infringe on the privacy of people by prosecuting developers which champion digital freedoms and seizing their infrastructure, Sparrow server has become somewhat obsolete. The above use case would allow a revival by acting as the client who pushes such events.

Sparrow Server could then act as a personal watch tower, for example detecting if a transaction appears in the mempool that has utxos of one of its wallets in it - which could mean that said wallet has been compromised (e.g. in the case of a watch only one). The notification would get pushed to your desktop or mobile client and you could check and possibly react in time. This would prevent a periodic check of your wallets for peace of mind.

The implementation I'm thinking of is ntfy. There is also a java library.

Sparrow Server would 'only' need the logic to specify when to push, the push notification content and a settings page where one could input the ntfy server, the authentication if so requried and the topic to which to send the http PUT/POST requests. There's even an existing mobile app, see all infos on the ntfy page.

Self-hosting a ntfy server is pretty easy. For those not able or not willing to, the notifications pushed to an 3th party server (e.g. ntfy.sh) could contain a more general, privacy preserving description ("Transaction detected in mempool").

ntfy servers could be exposed as hidden services and connections could be served over tor only to further increase privacy for non selfhosting users.