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

- Refactored KnowledgeUnit to use Guid EbookId and added navigation property
- Updated AppDbContext with fluent configuration and cascade delete
- Propagated ebookId through IKnowledgeService and API endpoints
- Generated EF migration for schema changes

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 =>