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/main.rs

@@ -38,6 +38,7 @@ mod network;
 mod nostr_relays;
 mod update;
 mod vpn;
+mod webhooks;
 
 use auth::AuthManager;
 use config::Config;