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.Web.Client/Services/WasmEpubService.cs

@@ -14,24 +14,26 @@ public class WasmEpubReader : IEpubReader
         _httpClient = httpClient;
     }
 
-    public async Task<Result<ReaderPageViewModel>> GetEpubContentAsync(int chapterIndex, string? userId = null)
+    public async Task<Result<ReaderPageViewModel>> GetEpubContentAsync(
+        Guid ebookId,
+        int chapterIndex,
+        string? userId = null,
+        CancellationToken cancellationToken = default)
     {
         try
         {
-            var response = await _httpClient.GetAsync($"/api/epub/{chapterIndex}");
+            var response = await _httpClient.GetAsync($"/api/epub/{ebookId}/{chapterIndex}", cancellationToken);
             if (response.IsSuccessStatusCode)
             {
-                var viewModel = await response.Content.ReadFromJsonAsync<ReaderPageViewModel>();
+                var viewModel = await response.Content.ReadFromJsonAsync<ReaderPageViewModel>(cancellationToken: cancellationToken);
                 return viewModel != null ? Result.Ok(viewModel) : Result.Fail("Failed to deserialize response.");
             }
-            
-            // Try to read the error message from the body
-            var errorBody = await response.Content.ReadAsStringAsync();
+
+            var errorBody = await response.Content.ReadAsStringAsync(cancellationToken);
             return Result.Fail($"Server error ({response.StatusCode}): {errorBody}");
         }
         catch (Exception ex)
         {
-            // Fallback for network errors or parsing exceptions
             return Result.Fail(new Error($"Network or parsing error: {ex.Message}").CausedBy(ex));
         }
     }