feat: add NODE_IDENTITY_PUBKEYS derived-env placeholder

Lets an app grant the node's users owner rights, e.g. a Blossom server's
allowed uploaders. The value is the Nostr keys of the identities the app
identity picker offers for NIP-07 signing, chosen by the same rule as
NostrIdentityPicker.vue, so the node's own appliance identity is never
included. It is resolved only for manifests that template it, and an
empty set is an error rather than an empty owner list.

identity.list now shares its is_node test with the new helper.

core/archipelago/src/api/rpc/identity/handlers.rs

@@ -1,6 +1,8 @@
 use super::*;
 use crate::api::rpc::RpcHandler;
-use crate::identity_manager::{IdentityManager, IdentityProfile, IdentityPurpose};
+use crate::identity_manager::{
+    is_node_identity, IdentityManager, IdentityProfile, IdentityPurpose,
+};
 use crate::network::did_dht;
 use anyhow::{Context, Result};
 use nostr_sdk::ToBech32;
@@ -38,7 +40,7 @@ impl RpcHandler {
             .into_iter()
             .map(|id| {
                 let is_default = default_id.as_deref() == Some(&id.id);
-                let is_node = !node_pubkey_hex.is_empty() && id.pubkey_hex == node_pubkey_hex;
+                let is_node = is_node_identity(&id, &node_pubkey_hex);
                 let (nostr_pubkey, nostr_npub) = if is_node {
                     (
                         node_nostr_hex.clone().or(id.nostr_pubkey),