feat: implement semantic search, knowledge unit extraction, and visualization components

src/NexusReader.Domain/Entities/KnowledgeUnit.cs

@@ -0,0 +1,41 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+using NexusReader.Domain.Enums;
+
+namespace NexusReader.Domain.Entities;
+
+public class KnowledgeUnit
+{
+    [Key]
+    [MaxLength(128)]
+    public string Id { get; set; } = string.Empty; // Hash(Source + Content + Version)
+
+    [Required]
+    [MaxLength(128)]
+    public string SourceId { get; set; } = string.Empty;
+
+    [Required]
+    [MaxLength(50)]
+    public string Version { get; set; } = "1.0";
+
+    [Required]
+    public KnowledgeUnitType Type { get; set; }
+
+    [Required]
+    public string Content { get; set; } = string.Empty;
+
+    public string? MetadataJson { get; set; } // e.g. { "page": 1, "path": "Chapter 1 > Intro" }
+
+    [Required]
+    [MaxLength(128)]
+    public string TenantId { get; set; } = string.Empty;
+
+    [Column(TypeName = "vector(768)")] // Default for text-embedding-004
+    public float[]? Vector { get; set; }
+
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    // Relationships
+    public ICollection<KnowledgeUnitLink> OutgoingLinks { get; set; } = new List<KnowledgeUnitLink>();
+    public ICollection<KnowledgeUnitLink> IncomingLinks { get; set; } = new List<KnowledgeUnitLink>();
+}