feat(ai): 向量搜索 + hit test API

- KnowledgeV2Service.vector_search: pgvector 余弦相似度搜索
- SearchHit DTO: chunk_id/document_id/similarity/metadata
- hit_test handler: POST /ai/documents/hit-test (embed query → 搜索)
- AiState 添加 embedding 字段，共享 EmbeddingService 实例
- top_k 限制最大 20

Phase 2 Task 11

crates/erp-server/src/main.rs

@@ -594,6 +594,13 @@ async fn main() -> anyhow::Result<()> {
             cache_ttl,
         ));
 
+        let embedding_svc = std::sync::Arc::new(
+            erp_ai::service::embedding::EmbeddingService::from_settings(&db).await,
+        );
+        let knowledge_v2_svc = std::sync::Arc::new(
+            erp_ai::service::knowledge_v2::KnowledgeV2Service::new(db.clone()),
+        );
+
         erp_ai::AiState {
             db: db.clone(),
             event_bus: event_bus.clone(),
@@ -612,13 +619,15 @@ async fn main() -> anyhow::Result<()> {
             ),
             knowledge: std::sync::Arc::new(erp_ai::service::knowledge::KnowledgeService::new(
                 db.clone(),
-                std::sync::Arc::new(
-                    erp_ai::service::embedding::EmbeddingService::from_settings(&db).await,
-                ),
+                embedding_svc.clone(),
             )),
-            knowledge_v2: std::sync::Arc::new(
-                erp_ai::service::knowledge_v2::KnowledgeV2Service::new(db.clone()),
-            ),
+            knowledge_v2: knowledge_v2_svc.clone(),
+            document: std::sync::Arc::new(erp_ai::service::document::DocumentService::new(
+                db.clone(),
+                knowledge_v2_svc,
+                embedding_svc.clone(),
+            )),
+            embedding: embedding_svc,
             chat_session: std::sync::Arc::new(
                 erp_ai::service::chat_session::ChatSessionService::new(db.clone()),
             ),