feat: add webhook notification system with Settings UI (REMOTE-03)

Webhook module with HTTP delivery, HMAC-SHA256 signing, and event
filtering. RPC handlers for get-config, configure, and test endpoints.
Settings page gains webhook configuration section with URL, secret,
event toggles, and test button.

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

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

@@ -26,6 +26,7 @@ mod system;
 mod update;
 mod vpn;
 mod wallet;
+mod webhooks;
 
 use crate::auth::AuthManager;
 use crate::config::Config;
@@ -490,6 +491,11 @@ impl RpcHandler {
                 self.handle_security_list_expiring(&p).await
             }
 
+            // Webhooks
+            "webhook.get-config" => self.handle_webhook_get_config().await,
+            "webhook.configure" => self.handle_webhook_configure(params).await,
+            "webhook.test" => self.handle_webhook_test().await,
+
             _ => {
                 Err(anyhow::anyhow!("Unknown method: {}", rpc_req.method))
             }