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.
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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>
