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.Web.Client/Services/WasmKnowledgeService.cs

@@ -14,24 +14,24 @@ public class WasmKnowledgeService : IKnowledgeService
         _httpClient = httpClient;
     }
 
-    public async Task<Result<KnowledgePacket>> GetKnowledgeAsync(string text, string tenantId, CancellationToken cancellationToken = default)
+    public async Task<Result<KnowledgePacket>> GetKnowledgeAsync(string text, string tenantId, Guid? ebookId = null, CancellationToken cancellationToken = default)
     {
-        return await CallKnowledgeApiAsync("/api/knowledge", text, cancellationToken);
+        return await CallKnowledgeApiAsync("/api/knowledge", text, ebookId, cancellationToken);
     }
 
-    public async Task<Result<KnowledgePacket>> GetGraphDataAsync(string text, string tenantId, CancellationToken cancellationToken = default)
+    public async Task<Result<KnowledgePacket>> GetGraphDataAsync(string text, string tenantId, Guid? ebookId = null, CancellationToken cancellationToken = default)
     {
-        return await CallKnowledgeApiAsync("/api/knowledge/graph", text, cancellationToken);
+        return await CallKnowledgeApiAsync("/api/knowledge/graph", text, ebookId, cancellationToken);
     }
 
-    public async Task<Result<KnowledgePacket>> GetKnowledgeMapAsync(string text, string tenantId, CancellationToken cancellationToken = default)
+    public async Task<Result<KnowledgePacket>> GetKnowledgeMapAsync(string text, string tenantId, Guid? ebookId = null, CancellationToken cancellationToken = default)
     {
-        return await CallKnowledgeApiAsync("/api/knowledge/map", text, cancellationToken);
+        return await CallKnowledgeApiAsync("/api/knowledge/map", text, ebookId, cancellationToken);
     }
 
-    public async Task<Result<KnowledgePacket>> GetSummaryAndQuizAsync(string text, string tenantId, CancellationToken cancellationToken = default)
+    public async Task<Result<KnowledgePacket>> GetSummaryAndQuizAsync(string text, string tenantId, Guid? ebookId = null, CancellationToken cancellationToken = default)
     {
-        return await CallKnowledgeApiAsync("/api/knowledge/summary", text, cancellationToken);
+        return await CallKnowledgeApiAsync("/api/knowledge/summary", text, ebookId, cancellationToken);
     }
 
     public async Task<Result<List<RelevantContext>>> GetRelevantContextAsync(string query, string tenantId, CancellationToken cancellationToken = default)
@@ -73,11 +73,11 @@ public class WasmKnowledgeService : IKnowledgeService
         }
     }
 
-    private async Task<Result<KnowledgePacket>> CallKnowledgeApiAsync(string endpoint, string text, CancellationToken cancellationToken)
+    private async Task<Result<KnowledgePacket>> CallKnowledgeApiAsync(string endpoint, string text, Guid? ebookId, CancellationToken cancellationToken)
     {
         try
         {
-            var response = await _httpClient.PostAsJsonAsync(endpoint, new { text }, cancellationToken);
+            var response = await _httpClient.PostAsJsonAsync(endpoint, new { text, ebookId }, cancellationToken);
             if (response.IsSuccessStatusCode)
             {
                 var packet = await response.Content.ReadFromJsonAsync<KnowledgePacket>(cancellationToken: cancellationToken);