feat(saas): add GenerateEmbedding worker for knowledge chunking

- Markdown-aware content splitting (512 token chunks with 64 overlap)
- CJK keyword extraction from chunk content with stop-word filtering
- Full refresh strategy (delete old chunks → re-insert on update)
- Phase 2 placeholder for vector embedding API integration

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

crates/zclaw-saas/src/workers/mod.rs

@@ -241,6 +241,7 @@ pub mod cleanup_refresh_tokens;
 pub mod update_last_used;
 pub mod record_usage;
 pub mod aggregate_usage;
+pub mod generate_embedding;
 
 // 便捷导出
 pub use log_operation::LogOperationWorker;