feat(knowledge): Phase A 知识库可见性隔离 + 结构化数据源 + 蒸馏Worker

- knowledge_items 增加 visibility(public/private) + account_id 字段
- 新建 structured_sources + structured_rows 表 (Excel JSONB 行级存储)
- 结构化数据源 CRUD API (5 路由: list/get/rows/delete/query)
- 安全查询: JSONB GIN 索引 + 可见性过滤 + 行数限制
- 蒸馏 Worker: 复用 Provider Key Pool 调 DeepSeek/Qwen API
- L0 质量过滤: 长度/隐私检测
- create_item 增加 is_admin 参数控制可见性默认值
- generate_embedding: extract_keywords_from_text 改为 pub 复用

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

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

@@ -251,6 +251,7 @@ pub mod update_last_used;
 pub mod record_usage;
 pub mod aggregate_usage;
 pub mod generate_embedding;
+pub mod distill_knowledge;
 
 // 便捷导出
 pub use log_operation::LogOperationWorker;
@@ -259,3 +260,4 @@ pub use cleanup_refresh_tokens::CleanupRefreshTokensWorker;
 pub use update_last_used::UpdateLastUsedWorker;
 pub use record_usage::RecordUsageWorker;
 pub use aggregate_usage::AggregateUsageWorker;
+pub use distill_knowledge::DistillationWorker;