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/Migrations/AppDbContextModelSnapshot.cs

@@ -247,14 +247,12 @@ namespace NexusReader.Data.Migrations
                     b.Property<DateTime>("CreatedAt")
                         .HasColumnType("timestamp with time zone");
 
+                    b.Property<Guid?>("EbookId")
+                        .HasColumnType("uuid");
+
                     b.Property<string>("MetadataJson")
                         .HasColumnType("text");
 
-                    b.Property<string>("SourceId")
-                        .IsRequired()
-                        .HasMaxLength(128)
-                        .HasColumnType("character varying(128)");
-
                     b.Property<string>("TenantId")
                         .IsRequired()
                         .HasMaxLength(128)
@@ -273,7 +271,7 @@ namespace NexusReader.Data.Migrations
 
                     b.HasKey("Id");
 
-                    b.HasIndex("SourceId");
+                    b.HasIndex("EbookId");
 
                     b.HasIndex("TenantId");
 
@@ -635,6 +633,16 @@ namespace NexusReader.Data.Migrations
                     b.Navigation("User");
                 });
 
+            modelBuilder.Entity("NexusReader.Domain.Entities.KnowledgeUnit", b =>
+                {
+                    b.HasOne("NexusReader.Domain.Entities.Ebook", "Ebook")
+                        .WithMany()
+                        .HasForeignKey("EbookId")
+                        .OnDelete(DeleteBehavior.Cascade);
+
+                    b.Navigation("Ebook");
+                });
+
             modelBuilder.Entity("NexusReader.Domain.Entities.KnowledgeUnitLink", b =>
                 {
                     b.HasOne("NexusReader.Domain.Entities.KnowledgeUnit", "SourceUnit")