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.Application/Abstractions/Services/IEpubReader.cs

@@ -3,7 +3,21 @@ using NexusReader.Application.Queries.Reader;
 
 namespace NexusReader.Application.Abstractions.Services;
 
+/// <summary>
+/// Reads and parses EPUB content for a specific ebook and chapter.
+/// </summary>
 public interface IEpubReader
 {
-    Task<Result<ReaderPageViewModel>> GetEpubContentAsync(int chapterIndex, string? userId = null);
+    /// <summary>
+    /// Retrieves the content blocks for a given chapter of the specified ebook.
+    /// </summary>
+    /// <param name="ebookId">The unique ID of the ebook to read.</param>
+    /// <param name="chapterIndex">Zero-based chapter index.</param>
+    /// <param name="userId">The authenticated user's ID (used for tenant isolation in the DB lookup).</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    Task<Result<ReaderPageViewModel>> GetEpubContentAsync(
+        Guid ebookId,
+        int chapterIndex,
+        string? userId = null,
+        CancellationToken cancellationToken = default);
 }