SECURITY: Encrypt payloads using target user's public key

# Wish

Encrypt messages and data streams that meant for a single, specific user using the target user's public pico.sh SSH key.

# Background

The pico.sh service supports querying the public keys of a specific user;

```sh
$ curl https://auth.pico.sh/pubkeys/alice
```

(This was added on 2026-01-22, cf. https://github.com/picosh/pico/issues/199#issuecomment-3977454340)

# Tasks

* [ ] Which key to use when a user have multiple public keys?
* [ ] Should user's announce their public keys in the message board part of their posted messages?
- [ ] only allow keys available on pico.sh
- [ ] maybe a short digest hash code, which is sufficient to identify the key in case there are multiple?
* [ ] Add a once-per-session key validation, i.e. verify that the select public key works and then memoize the (user, public key) map
* [ ] ...?