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/Persistence/IEbookRepository.cs

@@ -0,0 +1,30 @@
+using NexusReader.Domain.Entities;
+
+namespace NexusReader.Application.Abstractions.Persistence;
+
+/// <summary>
+/// Abstraction for Ebook and Author persistence operations.
+/// Defined in the Application layer to avoid a direct dependency on EF Core.
+/// </summary>
+public interface IEbookRepository
+{
+    /// <summary>
+    /// Finds an author by name using a case-insensitive comparison.
+    /// </summary>
+    Task<Author?> FindAuthorByNameAsync(string name, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Adds a new author to the repository (staged, not yet persisted).
+    /// </summary>
+    void AddAuthor(Author author);
+
+    /// <summary>
+    /// Adds a new ebook to the repository (staged, not yet persisted).
+    /// </summary>
+    void AddEbook(Ebook ebook);
+
+    /// <summary>
+    /// Persists all staged changes to the underlying store.
+    /// </summary>
+    Task<int> SaveChangesAsync(CancellationToken cancellationToken = default);
+}