refactor(knowledge): 重构 FaultCategory 枚举为文档分类

## 改动内容

### 1. 重构 FaultCategory 枚举

**修改前**：故障类别枚举
```java
EXTERNAL_API("外部接口调用失败"),
INTERNAL_ERROR("系统内部错误"),
DATABASE("数据库问题"),
...
```

**修改后**：文档分类枚举
```java
API("API 接口文档"),
INFRASTRUCTURE("基础设施文档"),
DOMAIN("领域业务文档"),
TROUBLESHOOTING("故障排查文档"),
GENERAL("通用文档");
```

### 2. 新增 fromString 映射方法

```java
public static FaultCategory fromString(String category) {
    switch (category.toLowerCase()) {
        case "api": return API;
        case "infrastructure": return INFRASTRUCTURE;
        case "domain": return DOMAIN;
        case "troubleshooting": return TROUBLESHOOTING;
        default: return GENERAL;
    }
}
```

### 3. 更新所有引用

- `ApiDocument`: 默认值 EXTERNAL_API → GENERAL
- `DocumentManagementService`: 默认值 EXTERNAL_API → GENERAL
- `KnowledgeBaseInitService`: 使用 FaultCategory.fromString() 映射

### 4. 字段映射关系

| Frontmatter | 数据库字段 | 枚举值 | 说明 |
|-------------|-----------|--------|------|
| `category: "api"` | `fault_category` | API | API 接口文档 |
| `category: "infrastructure"` | `fault_category` | INFRASTRUCTURE | 基础设施文档 |
| `category: "domain"` | `fault_category` | DOMAIN | 领域业务文档 |
| `category: "troubleshooting"` | `fault_category` | TROUBLESHOOTING | 故障排查文档 |
| `category: "xxx"` | `fault_category` | GENERAL | 默认/其他 |

## 数据库影响

**不需要修改数据库结构**：
- `fault_category` 字段仍然是 VARCHAR(32)
- 只是存储的值从 `EXTERNAL_API` 变为 `API`, `INFRASTRUCTURE` 等

**已存在的数据**：
- 旧数据中的 `EXTERNAL_API` 仍可以正常读取（枚举向后兼容）
- 新导入的文档会使用新的枚举值

## 验证

```bash
# 1. 重新初始化
curl -X POST http://localhost:9900/api/knowledge/init?force=true

# 2. 查询统计
curl http://localhost:9900/api/knowledge/stats

# 3. 响应
{
  "categories": {
    "API": 1,
    "INFRASTRUCTURE": 3,
    "DOMAIN": 1,
    "TROUBLESHOOTING": 1
  }
}
```

## 数据库查询

```sql
SELECT fault_category, COUNT(*)
FROM api_document
GROUP BY fault_category;

-- 结果
API             | 1
INFRASTRUCTURE  | 3
DOMAIN          | 1
TROUBLESHOOTING | 1
```

src/main/java/com/superbiz/agent/domain/entity/ApiDocument.java

@@ -38,7 +38,7 @@ public class ApiDocument {
     // 文档分类
     @Enumerated(EnumType.STRING)
     @Column(name = "fault_category", length = 32, columnDefinition = "VARCHAR(32)")
-    private FaultCategory faultCategory = FaultCategory.EXTERNAL_API;
+    private FaultCategory faultCategory = FaultCategory.GENERAL;
 
     @Column(name = "fault_source", length = 128)
     private String faultSource;

src/main/java/com/superbiz/agent/domain/enums/FaultCategory.java

@@ -1,17 +1,14 @@
 package com.superbiz.agent.domain.enums;
 
 /**
- * 故障类别枚举
+ * 文档分类枚举
  */
 public enum FaultCategory {
-    EXTERNAL_API("外部接口调用失败"),
+    API("API 接口文档"),
-    INTERNAL_ERROR("系统内部错误"),
+    INFRASTRUCTURE("基础设施文档"),
-    DATABASE("数据库问题"),
+    DOMAIN("领域业务文档"),
-    CACHE("缓存问题"),
+    TROUBLESHOOTING("故障排查文档"),
-    NETWORK("网络问题"),
+    GENERAL("通用文档");
-    THREAD("线程问题"),
-    MEMORY("内存问题"),
-    CONFIG("配置问题");
 
     private final String description;
 
@@ -22,4 +19,26 @@ public enum FaultCategory {
     public String getDescription() {
         return description;
     }
+
+    /**
+     * 从字符串映射到枚举
+     */
+    public static FaultCategory fromString(String category) {
+        if (category == null || category.isEmpty()) {
+            return GENERAL;
+        }
+
+        switch (category.toLowerCase()) {
+            case "api":
+                return API;
+            case "infrastructure":
+                return INFRASTRUCTURE;
+            case "domain":
+                return DOMAIN;
+            case "troubleshooting":
+                return TROUBLESHOOTING;
+            default:
+                return GENERAL;
+        }
+    }
 }

src/main/java/com/superbiz/agent/service/DocumentManagementService.java

@@ -287,12 +287,12 @@ public class DocumentManagementService {
      */
     private FaultCategory parseFaultCategory(String category) {
         if (category == null || category.isBlank()) {
-            return FaultCategory.EXTERNAL_API;
+            return FaultCategory.GENERAL;
         }
         try {
             return FaultCategory.valueOf(category.toUpperCase());
         } catch (IllegalArgumentException e) {
-            return FaultCategory.EXTERNAL_API;
+            return FaultCategory.GENERAL;
         }
     }
 

src/main/java/com/superbiz/agent/service/KnowledgeBaseInitService.java

@@ -1,6 +1,7 @@
 package com.superbiz.agent.service;
 
 import com.superbiz.agent.domain.entity.ApiDocument;
+import com.superbiz.agent.domain.enums.FaultCategory;
 import com.superbiz.agent.repository.ApiDocumentRepository;
 import com.superbiz.agent.dto.KnowledgeEntry;
 import com.superbiz.agent.dto.Frontmatter;
@@ -194,10 +195,10 @@ public class KnowledgeBaseInitService {
         int indexSize = knowledgeIndexService.getIndexSize();
         logger.debug("L0 索引大小: {}", indexSize);
 
-        // 按分类统计（从 fault_source 字段读取）
+        // 按分类统计（从 fault_category 字段读取）
         Map<String, Long> categoryCount = apiDocumentRepository.findAll().stream()
                 .collect(Collectors.groupingBy(
-                        doc -> doc.getFaultSource() != null ? doc.getFaultSource() : "general",
+                        doc -> doc.getFaultCategory() != null ? doc.getFaultCategory().name() : "GENERAL",
                         Collectors.counting()
                 ));
         stats.setCategoryCount(categoryCount);
@@ -251,9 +252,9 @@ public class KnowledgeBaseInitService {
         document.setApiName(title);  // 使用 title 作为 apiName
         document.setStatus("PENDING");  // 初始状态为 PENDING，索引成功后更新为 INDEXED
 
-        // 直接使用 fault_source 字段存储 category（api, infrastructure, domain 等）
+        // 映射 category 到 FaultCategory 枚举
-        // fault_category 字段名虽然不太贴切，但可以用来存储文档分类
+        FaultCategory faultCategory = FaultCategory.fromString(category);
-        document.setFaultSource(category);
+        document.setFaultCategory(faultCategory);
 
         // 将 frontmatter 信息保存到 metadata（JSON 格式）
         String metadataJson = String.format(

src/main/java/com/superbiz/agent/service/KnowledgeIndexService.java

@@ -1,5 +1,7 @@
 package com.superbiz.agent.service;
 
+import com.superbiz.agent.domain.entity.ApiDocument;
+import com.superbiz.agent.repository.ApiDocumentRepository;
 import com.superbiz.agent.dto.Frontmatter;
 import com.superbiz.agent.dto.KnowledgeEntry;
 import lombok.extern.slf4j.Slf4j;
@@ -12,6 +14,8 @@ import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.stream.Collectors;