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/AppDbContext.cs

@@ -65,8 +65,13 @@ public class AppDbContext : IdentityDbContext<NexusUser>
         {
             entity.HasKey(e => e.Id);
             entity.HasIndex(e => e.TenantId);
-            entity.HasIndex(e => e.SourceId);
+            entity.HasIndex(e => e.EbookId);
             entity.Property(e => e.Vector).HasColumnType("vector(768)");
+
+            entity.HasOne(e => e.Ebook)
+                .WithMany()
+                .HasForeignKey(e => e.EbookId)
+                .OnDelete(DeleteBehavior.Cascade);
         });
 
         modelBuilder.Entity<KnowledgeUnitLink>(entity =>