feat(knowledge): Phase D 统一搜索 + 种子知识冷启动

- search/recommend API 返回 UnifiedSearchResult (文档+结构化双通道)
- POST /api/v1/knowledge/seed 种子知识冷启动 (幂等, admin权限)
- seed_knowledge service: 按标题+行业查重, source=distillation
- SearchRequest 扩展: search_structured/search_documents/industry_id

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

@@ -32,6 +32,7 @@ pub fn routes() -> axum::Router<crate::state::AppState> {
         // 检索
         .route("/api/v1/knowledge/search", post(handlers::search))
         .route("/api/v1/knowledge/recommend", post(handlers::recommend))
+        .route("/api/v1/knowledge/seed", post(handlers::seed_knowledge))
         // 分析看板
         .route("/api/v1/knowledge/analytics/overview", get(handlers::analytics_overview))
         .route("/api/v1/knowledge/analytics/trends", get(handlers::analytics_trends))