refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/internal/dto/passkey.go

@@ -0,0 +1,11 @@
+package dto
+
+type Passkey struct {
+	ID         int64  `json:"id" gorm:"column:id;primaryKey;autoIncrement"`
+	Name       string `json:"name" gorm:"column:name"`                 // 凭证名称，用于用户识别不同的设备
+	SignCount  uint32 `json:"sign_count" gorm:"column:sign_count"`     // 签名计数器，用于防止重放攻击
+	DeviceType string `json:"device_type" gorm:"column:device_type"`   // 设备类型，如"platform"或"cross-platform"
+	LastUsedAt int64  `json:"last_used_at" gorm:"column:last_used_at"` // 最后使用时间
+	CreatedAt  int64  `json:"created_at,omitempty" gorm:"autoCreateTime"`
+	UpdatedAt  int64  `json:"updated_at,omitempty" gorm:"autoUpdateTime"`
+}