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.Domain/Entities/KnowledgeUnit.cs

@@ -11,9 +11,10 @@ public class KnowledgeUnit
     [MaxLength(128)]
     public string Id { get; set; } = string.Empty; // Hash(Source + Content + Version)
 
-    [Required]
-    [MaxLength(128)]
-    public string SourceId { get; set; } = string.Empty;
+    public Guid? EbookId { get; set; }
+ 
+     [ForeignKey(nameof(EbookId))]
+    public virtual Ebook? Ebook { get; set; }
 
     [Required]
     [MaxLength(50)]