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-ai/src/service/knowledge_v2.rs

@@ -255,4 +255,82 @@ impl KnowledgeV2Service {
             .map_err(|e| AiError::DbError(e.to_string()))?;
         Ok(())
     }
+
+    /// 向量相似度搜索：在指定知识库中搜索与 query_embedding 最相似的 top_k 个切片
+    pub async fn vector_search(
+        &self,
+        tenant_id: Uuid,
+        kb_id: Uuid,
+        query_embedding: &[f32],
+        top_k: i64,
+    ) -> AiResult<Vec<SearchHit>> {
+        let vector_str = crate::service::embedding::format_vector(query_embedding);
+        let sql = r#"
+            SELECT c.id, c.document_id, c.chunk_index, c.content, c.metadata,
+                   d.title AS doc_title,
+                   1 - (c.embedding <=> $3::vector) AS similarity
+            FROM ai_knowledge_chunks c
+            JOIN ai_knowledge_documents d ON d.id = c.document_id
+            WHERE c.tenant_id = $1
+              AND c.knowledge_base_id = $2
+              AND c.deleted_at IS NULL
+              AND d.deleted_at IS NULL
+              AND c.embedding IS NOT NULL
+            ORDER BY c.embedding <=> $3::vector
+            LIMIT $4
+        "#;
+        let stmt = sea_orm::Statement::from_sql_and_values(
+            sea_orm::DatabaseBackend::Postgres,
+            sql,
+            [
+                sea_orm::Value::from(tenant_id),
+                sea_orm::Value::from(kb_id),
+                sea_orm::Value::String(Some(Box::new(vector_str))),
+                sea_orm::Value::from(top_k),
+            ],
+        );
+
+        let rows: Vec<SearchHitRow> = sea_orm::FromQueryResult::find_by_statement(stmt)
+            .all(&self.db)
+            .await
+            .map_err(|e| AiError::DbError(e.to_string()))?;
+
+        Ok(rows.into_iter().map(SearchHit::from).collect())
+    }
+}
+
+#[derive(Debug, sea_orm::FromQueryResult)]
+struct SearchHitRow {
+    id: Uuid,
+    document_id: Uuid,
+    chunk_index: i32,
+    content: String,
+    metadata: serde_json::Value,
+    doc_title: String,
+    similarity: f64,
+}
+
+#[derive(Debug, serde::Serialize)]
+pub struct SearchHit {
+    pub chunk_id: Uuid,
+    pub document_id: Uuid,
+    pub chunk_index: i32,
+    pub content: String,
+    pub doc_title: String,
+    pub similarity: f64,
+    pub metadata: serde_json::Value,
+}
+
+impl From<SearchHitRow> for SearchHit {
+    fn from(row: SearchHitRow) -> Self {
+        Self {
+            chunk_id: row.id,
+            document_id: row.document_id,
+            chunk_index: row.chunk_index,
+            content: row.content,
+            doc_title: row.doc_title,
+            similarity: row.similarity,
+            metadata: row.metadata,
+        }
+    }
 }