feat(admin): Implement System Knowledge Base management module

Features:

- Backend: SystemKbService with full CRUD (knowledge bases + documents)

- Backend: 8 RESTful API endpoints (list/detail/create/update/delete/upload/download)

- Backend: OSS storage integration (system/knowledge-bases/{kbId}/{docId})

- Backend: RAG engine integration (document parsing, chunking, vectorization)

- Frontend: SystemKbListPage with card-based layout

- Frontend: SystemKbDetailPage with document management table

- Frontend: Master-Detail UX pattern for better user experience

- Document upload (single/batch), download (preserving original filename), delete

Technical:

- Database migration for system_knowledge_bases and system_kb_documents tables

- OSSAdapter.getSignedUrl with Content-Disposition for original filename

- Reuse RAG engine from common/rag for document processing

Tested: Local environment verified, all features working

backend/src/common/storage/OSSAdapter.ts

@@ -91,10 +91,18 @@ export class OSSAdapter implements StorageAdapter {
     try {
       const normalizedKey = this.normalizeKey(key)
       
-      // 使用 put 方法上传 Buffer（适合 < 30MB 文件）
-      const result = await this.client.put(normalizedKey, buffer)
+      // 根据文件扩展名设置 Content-Type
+      const contentType = this.getMimeType(normalizedKey)
       
-      console.log(`[OSSAdapter] Upload success: ${normalizedKey}, size=${buffer.length}`)
+      // 使用 put 方法上传 Buffer（适合 < 30MB 文件）
+      // 必须设置正确的 headers，否则二进制文件可能损坏
+      const result = await this.client.put(normalizedKey, buffer, {
+        headers: {
+          'Content-Type': contentType,
+        },
+      })
+      
+      console.log(`[OSSAdapter] Upload success: ${normalizedKey}, size=${buffer.length}, contentType=${contentType}`)
       
       // 返回签名URL（假设是私有Bucket）
       return this.getSignedUrl(normalizedKey)
@@ -104,6 +112,37 @@ export class OSSAdapter implements StorageAdapter {
     }
   }
 
+  /**
+   * 根据文件扩展名获取 MIME 类型
+   */
+  private getMimeType(key: string): string {
+    const ext = key.toLowerCase().split('.').pop()
+    const mimeTypes: Record<string, string> = {
+      // 文档类型
+      pdf: 'application/pdf',
+      doc: 'application/msword',
+      docx: 'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
+      xls: 'application/vnd.ms-excel',
+      xlsx: 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
+      ppt: 'application/vnd.ms-powerpoint',
+      pptx: 'application/vnd.openxmlformats-officedocument.presentationml.presentation',
+      txt: 'text/plain',
+      md: 'text/markdown',
+      // 图片类型
+      jpg: 'image/jpeg',
+      jpeg: 'image/jpeg',
+      png: 'image/png',
+      gif: 'image/gif',
+      webp: 'image/webp',
+      svg: 'image/svg+xml',
+      // 其他
+      json: 'application/json',
+      xml: 'application/xml',
+      zip: 'application/zip',
+    }
+    return mimeTypes[ext || ''] || 'application/octet-stream'
+  }
+
   /**
    * 流式上传（适合大文件 > 30MB）
    *