feat: establish formal relationship between KnowledgeUnit and Ebook (#35) (#43)

This PR finalizes the implementation of issue #35 by establishing a formal foreign key relationship between `KnowledgeUnit` and `Ebook`.

Closes #35

### Changes:
- **Domain**: Refactored `KnowledgeUnit` to use `Guid EbookId` and added navigation property.
- **Data**: Updated `AppDbContext` fluent configuration and generated a new migration.
- **Service**: Updated `IKnowledgeService` and its implementations to propagate `ebookId`.
- **API**: Updated Web API endpoints to support linking extracted knowledge to specific ebooks.

### Verification:
- [x] Solution builds successfully (`dotnet build`).
- [x] Schema changes verified in migration file.
- [x] Cascading delete behavior confirmed.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #43
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.Data/Persistence/Migrations/20260513185108_MakeKnowledgeUnitEbookIdNullable.cs

@@ -0,0 +1,37 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace NexusReader.Data.Persistence.Migrations
+{
+    /// <inheritdoc />
+    public partial class MakeKnowledgeUnitEbookIdNullable : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterColumn<Guid>(
+                name: "EbookId",
+                table: "KnowledgeUnits",
+                type: "uuid",
+                nullable: true,
+                oldClrType: typeof(Guid),
+                oldType: "uuid");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterColumn<Guid>(
+                name: "EbookId",
+                table: "KnowledgeUnits",
+                type: "uuid",
+                nullable: false,
+                defaultValue: new Guid("00000000-0000-0000-0000-000000000000"),
+                oldClrType: typeof(Guid),
+                oldType: "uuid",
+                oldNullable: true);
+        }
+    }
+}