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.Application/Queries/Graph/GetKnowledgeGraphQuery.cs

@@ -4,5 +4,6 @@ namespace NexusReader.Application.Queries.Graph;
 
 /// <param name="Text">Chapter or page content to extract the graph from.</param>
 /// <param name="TenantId">Tenant scope for knowledge extraction and caching.</param>
-public record GetKnowledgeGraphQuery(string Text, string TenantId) : IQuery<GraphDataDto>;
+/// <param name="EbookId">Optional Ebook ID to link the knowledge units to.</param>
+public record GetKnowledgeGraphQuery(string Text, string TenantId, Guid? EbookId = null) : IQuery<GraphDataDto>;
 

src/NexusReader.Application/Queries/Graph/GetKnowledgeGraphQueryHandler.cs

@@ -18,7 +18,11 @@ internal sealed class GetKnowledgeGraphQueryHandler : IQueryHandler<GetKnowledge
         if (string.IsNullOrWhiteSpace(request.Text))
             return Result.Ok(new GraphDataDto());
 
-        var result = await _knowledgeService.GetGraphDataAsync(request.Text, request.TenantId, cancellationToken);
+        var result = await _knowledgeService.GetGraphDataAsync(
+            request.Text, 
+            request.TenantId, 
+            ebookId: request.EbookId, 
+            cancellationToken: cancellationToken);
 
         if (result.IsFailed)
             return Result.Fail<GraphDataDto>(result.Errors);