feat: 完善知识库后端 API

- 添加 KnowledgeHandler 处理知识库请求
- 注册知识库 CRUD 路由
- 添加文档上传、删除、解析、预览接口
- 更新数据库模型和迁移

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/internal/model/knowledge_info.go

@@ -57,6 +57,7 @@ type KnowledgeDocument struct {
 	KnowledgeBaseID string    `json:"knowledge_base_id" gorm:"type:varchar(36);not null;index"`
 	Name             string    `json:"name" gorm:"type:varchar(255);not null"`
 	FileKey          string    `json:"file_key" gorm:"type:varchar(500)"`
+	FileURL          string    `json:"file_url" gorm:"type:varchar(500)"`  // 文件访问 URL
 	FileSize         int64     `json:"file_size" gorm:"type:bigint;default:0"`
 	Status           string    `json:"status" gorm:"type:varchar(20);default:parsing"` // parsing / parsed / failed
 	ChunkCount       int       `json:"chunk_count" gorm:"default:0"`