feat: Ingestion Pipeline Stabilization and WASM Service Proxies (#42)

This PR stabilizes the Nexus Ingestion Engine by implementing functional service proxies for the Blazor WASM client and refining the backend infrastructure for real-time progress tracking and database compatibility.

### Key Changes
- **Infrastructure Stabilization**:
  - Implemented production-grade `EbookRepository` with PostgreSQL `EF.Functions.ILike` support.
  - Enforced `IsReadyForReading = false` state for newly added ebooks (resolves #35).
  - Updated `SignalRSyncBroadcaster` to support targeted user messaging and ingestion-specific progress updates (resolves #37).
- **WASM Client Functional Proxies**:
  - Replaced "Throwing" dummy services with `WasmEbookRepository`, `WasmSyncBroadcaster`, `WasmBookStorageService`, and `WasmEmbeddingGenerator`.
  - These services proxy requests to the backend via a new set of Minimal API endpoints in `NexusReader.Web`.
- **Domain Refinement**:
  - Added `IsReadyForReading` flag to the `Ebook` entity to manage background AI processing states.

### Related Issues
- Fixes #35
- Fixes #36
- Fixes #37

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #42
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.UI.Shared/Services/KnowledgeCoordinator.cs

@@ -17,7 +17,11 @@ public sealed partial class KnowledgeCoordinator : IDisposable
     private readonly IReaderInteractionService _interactionService;
     private readonly ILogger<KnowledgeCoordinator> _logger;
     
-    public event Action<GraphDataDto>? OnGraphUpdated;
+    /// <summary>
+    /// Raised when the knowledge graph has been updated with new data.
+    /// Subscribers must return a Task to enable proper async handling.
+    /// </summary>
+    public event Func<GraphDataDto, Task>? OnGraphUpdated;
 
     public KnowledgeCoordinator(
         IKnowledgeService knowledgeService,
@@ -61,7 +65,8 @@ public sealed partial class KnowledgeCoordinator : IDisposable
                 if (packet.Graph != null)
                 {
                     await _graphService.UpdateGraph(packet.Graph);
-                    OnGraphUpdated?.Invoke(packet.Graph);
+                    if (OnGraphUpdated != null)
+                        await OnGraphUpdated.Invoke(packet.Graph);
                     await _platformService.VibrateSuccessAsync();
                 }
             }