Function publish_message 

pub async fn publish_message<T: Transport + ?Sized>(
    transport: &T,
    community: &Community,
    channel: &Channel,
    author: &Keys,
    content: &str,
    ms: u64,
) -> Result<(Event, Keys), String>

Seal content and publish it to the Community’s relays.

Returns the published outer event AND its retained ephemeral signing key. The key is one-time on the wire (no persistent author↔channel linkage), but the sender keeps it so they can later delete_own_message their own message Persist it (see db::community::store_message_key) — exactly like Vector’s nip17_wrap_keys for DMs. Discarding it just means that message can’t be deleted.