feat(message): add cross-module event integration (Phase 6)

- Message module subscribes to workflow events (process_instance.started)
- Auto-generates notifications when workflows start
- Added started_by to workflow instance event payload
- Event listener runs as background tokio task

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

crates/erp-server/src/main.rs

@@ -124,6 +124,10 @@ async fn main() -> anyhow::Result<()> {
     // Register event handlers
     registry.register_handlers(&event_bus);
 
+    // Start message event listener (workflow events → message notifications)
+    erp_message::MessageModule::start_event_listener(db.clone(), event_bus.clone());
+    tracing::info!("Message event listener started");
+
     let host = config.server.host.clone();
     let port = config.server.port;