Second checkpoint

internal/domain/category.go

@@ -0,0 +1,18 @@
+package domain
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+// Category is a hierarchical grouping for datasets. A category may have a parent
+// category (nil for a root) and many child categories.
+type Category struct {
+	ID          uuid.UUID  `json:"id"`
+	ParentID    *uuid.UUID `json:"parent_id"`
+	Name        string     `json:"name"`
+	Description string     `json:"description"`
+	CreatedAt   time.Time  `json:"created_at"`
+	UpdatedAt   time.Time  `json:"updated_at"`
+}