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.Infrastructure/Services/KnowledgeService.cs

@@ -50,27 +50,27 @@ public class KnowledgeService : IKnowledgeService
         _tokenizer = TiktokenTokenizer.CreateForModel("gpt-4");
     }
 
-    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 GetKnowledgeInternalAsync(text, tenantId, PromptRegistry.KnowledgeExtractionSystemPrompt, "full", cancellationToken);
+        return await GetKnowledgeInternalAsync(text, tenantId, PromptRegistry.KnowledgeExtractionSystemPrompt, "full", 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 GetKnowledgeInternalAsync(text, tenantId, PromptRegistry.GraphExtractionPrompt, "graph", cancellationToken);
+        return await GetKnowledgeInternalAsync(text, tenantId, PromptRegistry.GraphExtractionPrompt, "graph", 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 GetKnowledgeInternalAsync(text, tenantId, PromptRegistry.SummaryAndQuizPrompt, "summary_quiz", cancellationToken);
+        return await GetKnowledgeInternalAsync(text, tenantId, PromptRegistry.SummaryAndQuizPrompt, "summary_quiz", 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 GetKnowledgeInternalAsync(text, tenantId, PromptRegistry.KM_ExtractionPrompt, "km_map", cancellationToken);
+        return await GetKnowledgeInternalAsync(text, tenantId, PromptRegistry.KM_ExtractionPrompt, "km_map", ebookId, cancellationToken);
     }
 
-    private async Task<Result<KnowledgePacket>> GetKnowledgeInternalAsync(string text, string tenantId, string systemPrompt, string traceType, CancellationToken cancellationToken)
+    private async Task<Result<KnowledgePacket>> GetKnowledgeInternalAsync(string text, string tenantId, string systemPrompt, string traceType, Guid? ebookId, CancellationToken cancellationToken)
     {
         if (string.IsNullOrWhiteSpace(text)) return Result.Fail("Input text is empty.");
 
@@ -161,7 +161,7 @@ public class KnowledgeService : IKnowledgeService
                 }
 
                 // 5. Process structured KnowledgeUnits (Graph Expansion)
-                await ProcessKnowledgeUnitsAsync(knowledgePacket, tenantId, dbContext, cancellationToken);
+                await ProcessKnowledgeUnitsAsync(knowledgePacket, tenantId, ebookId, dbContext, cancellationToken);
 
                 await dbContext.SaveChangesAsync(cancellationToken);
                 return Result.Ok(knowledgePacket);
@@ -178,7 +178,7 @@ public class KnowledgeService : IKnowledgeService
         }
     }
 
-    private async Task ProcessKnowledgeUnitsAsync(KnowledgePacket packet, string tenantId, AppDbContext dbContext, CancellationToken cancellationToken)
+    private async Task ProcessKnowledgeUnitsAsync(KnowledgePacket packet, string tenantId, Guid? ebookId, AppDbContext dbContext, CancellationToken cancellationToken)
     {
         var unitIds = packet.Units.Select(u => u.Id).ToList();
         var linkSourceIds = packet.Links.Select(l => l.Source).ToList();
@@ -207,7 +207,11 @@ public class KnowledgeService : IKnowledgeService
 
             unit.Type = Enum.TryParse<NexusReader.Domain.Enums.KnowledgeUnitType>(unitDto.Type, true, out var type) ? type : NexusReader.Domain.Enums.KnowledgeUnitType.Snippet;
             unit.Content = unitDto.Content;
-            unit.SourceId = "extracted";
+            
+            // Link to the specific ebook if provided. 
+            // Link to ebook if provided
+            unit.EbookId = ebookId; 
+            
             unit.MetadataJson = JsonSerializer.Serialize(unitDto.Metadata);
 
             try