refactor(api): enforce protocol-neutral service boundaries

- refactor: move HTTP status and DTO concerns out of model and service
  layers into API and handler code.
- feat: add admin service boundary and route auth through services
  instead of direct repository access.
- test: add architecture and error tests covering package boundaries,
  redaction, and service behavior.
- docs: record the protocol-neutral boundary decision and update
  architecture and roadmap notes.

internal/model/file.go

@@ -9,16 +9,16 @@ const StatusUserDeleted = "user_deleted"
 
 // File represents a file or directory entry in the virtual filesystem.
 type File struct {
-	ID          string    `gorm:"primaryKey;type:varchar(36)" json:"id"`
-	UserID      string    `gorm:"index;uniqueIndex:idx_user_parent_name,priority:1;type:varchar(36);not null" json:"user_id"`
-	ParentID    *string   `gorm:"index;uniqueIndex:idx_user_parent_name,priority:2;type:varchar(36)" json:"parent_id"`
-	Name        string    `gorm:"type:varchar(255);not null;uniqueIndex:idx_user_parent_name,priority:3" json:"name"`
-	Size        int64     `gorm:"default:0" json:"size"`
-	MimeType    string    `gorm:"type:varchar(127)" json:"mime_type"`
-	StoragePath string    `gorm:"type:varchar(512)" json:"storage_path"`
-	Hash        string    `gorm:"type:varchar(64)" json:"-"`
-	Status      string    `gorm:"type:varchar(32);not null;default:active;index" json:"-"`
-	IsDir       bool      `gorm:"default:false" json:"is_dir"`
-	CreatedAt   time.Time `json:"created_at"`
-	UpdatedAt   time.Time `json:"updated_at"`
+	ID          string  `gorm:"primaryKey;type:varchar(36)"`
+	UserID      string  `gorm:"index;uniqueIndex:idx_user_parent_name,priority:1;type:varchar(36);not null"`
+	ParentID    *string `gorm:"index;uniqueIndex:idx_user_parent_name,priority:2;type:varchar(36)"`
+	Name        string  `gorm:"type:varchar(255);not null;uniqueIndex:idx_user_parent_name,priority:3"`
+	Size        int64   `gorm:"default:0"`
+	MimeType    string  `gorm:"type:varchar(127)"`
+	StoragePath string  `gorm:"type:varchar(512)" json:"-"`
+	Hash        string  `gorm:"type:varchar(64)" json:"-"`
+	Status      string  `gorm:"type:varchar(32);not null;default:active;index" json:"-"`
+	IsDir       bool    `gorm:"default:false"`
+	CreatedAt   time.Time
+	UpdatedAt   time.Time
 }