fix(saas): WorkerDispatcher registration race — consumer starts after all workers registered

Root cause: start_consumer() was called in new() before any register() calls,
so the consumer's cloned HashMap was always empty. Workers like log_operation
and record_usage were never found, causing "Unknown worker" errors.

- Add WorkerDispatcher::start() method to be called after all register()s
- Update main.rs to call dispatcher.start() after 7 workers registered

crates/zclaw-saas/src/main.rs

@@ -48,6 +48,7 @@ async fn main() -> anyhow::Result<()> {
     dispatcher.register(UpdateLastUsedWorker);
     dispatcher.register(AggregateUsageWorker);
     dispatcher.register(GenerateEmbeddingWorker);
+    dispatcher.start(); // 必须在所有 register() 之后调用
     info!("Worker dispatcher initialized (7 workers registered)");
 
     // 优雅停机令牌 — 取消后所有 SSE 流和长连接立即终止