feat: propagate Tor address rotation to Nostr relays and federation peers

After rotation, spawns background task that publishes updated .onion to
Nostr relays and sends federation.peer-address-changed RPC to all peers
over Tor. Peers update their nodes.json with the new address.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/archipelago/src/api/rpc/federation.rs

@@ -323,4 +323,46 @@ impl RpcHandler {
         info!(app = %app_id, peer = %peer_did, "Deployed app to federated peer");
         Ok(result)
     }
+
+    /// federation.peer-address-changed — A peer notifies us that their .onion changed.
+    pub(super) async fn handle_federation_peer_address_changed(
+        &self,
+        params: Option<serde_json::Value>,
+    ) -> Result<serde_json::Value> {
+        let params = params.ok_or_else(|| anyhow::anyhow!("Missing params"))?;
+        let did = params
+            .get("did")
+            .and_then(|v| v.as_str())
+            .ok_or_else(|| anyhow::anyhow!("Missing did"))?;
+        let new_onion = params
+            .get("new_onion")
+            .and_then(|v| v.as_str())
+            .ok_or_else(|| anyhow::anyhow!("Missing new_onion"))?;
+
+        // Load existing nodes, find the peer by DID, update their onion
+        let mut nodes = federation::load_nodes(&self.config.data_dir).await?;
+        let found = nodes.iter_mut().find(|n| n.did == did);
+
+        match found {
+            Some(node) => {
+                let old = node.onion.clone();
+                node.onion = new_onion.to_string();
+                federation::save_nodes(&self.config.data_dir, &nodes).await?;
+                info!(did = %did, old_onion = %old, new_onion = %new_onion, "Updated federated peer address");
+                Ok(serde_json::json!({
+                    "updated": true,
+                    "did": did,
+                    "old_onion": old,
+                    "new_onion": new_onion,
+                }))
+            }
+            None => {
+                info!(did = %did, "Received address change from unknown peer — ignoring");
+                Ok(serde_json::json!({
+                    "updated": false,
+                    "reason": "Unknown peer DID",
+                }))
+            }
+        }
+    }
 }