feat: enforce tenant isolation and shared question bank

Tiku.Domain/Catalog/TaxonomyEntities.cs

@@ -0,0 +1,37 @@
+using System.Text.Json;
+using Tiku.Domain.Common;
+
+namespace Tiku.Domain.Catalog;
+
+public sealed class TaxonomyNode : AuditableTenantEntity
+{
+    public Guid? ParentOwnerTenantId { get; set; }
+    public Guid? ParentId { get; set; }
+    public TaxonomyNodeType NodeType { get; set; } = TaxonomyNodeType.KnowledgePoint;
+    public string Code { get; set; } = string.Empty;
+    public string Name { get; set; } = string.Empty;
+    public string? Path { get; set; }
+    public int Depth { get; set; }
+    public int SortOrder { get; set; }
+    public bool IsActive { get; set; } = true;
+    public JsonElement Metadata { get; set; } = JsonDefaults.Object();
+}
+
+public sealed class QuestionTaxonomyAssignment : Entity, ITenantOwned
+{
+    public Guid TenantId { get; set; }
+    public Guid QuestionId { get; set; }
+    public Guid TaxonomyOwnerTenantId { get; set; }
+    public Guid TaxonomyNodeId { get; set; }
+    public bool IsPrimary { get; set; }
+    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+}
+
+public enum TaxonomyNodeType
+{
+    Subject,
+    Chapter,
+    KnowledgePoint,
+    Paper,
+    Custom
+}