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.Infrastructure/Persistence/EbookRepository.cs

@@ -0,0 +1,52 @@
+using Microsoft.EntityFrameworkCore;
+using NexusReader.Application.Abstractions.Persistence;
+using NexusReader.Data.Persistence;
+using NexusReader.Domain.Entities;
+
+namespace NexusReader.Infrastructure.Persistence;
+
+/// <summary>
+/// EF Core implementation of <see cref="IEbookRepository"/>.
+/// Uses a scoped <see cref="AppDbContext"/> created via the factory for long-running operations.
+/// </summary>
+internal sealed class EbookRepository : IEbookRepository
+{
+    private readonly AppDbContext _context;
+
+    public EbookRepository(AppDbContext context)
+    {
+        _context = context;
+    }
+
+    /// <inheritdoc />
+    public async Task<Author?> FindAuthorByNameAsync(string name, CancellationToken cancellationToken = default)
+    {
+        // Use PostgreSQL ILike for case-insensitive searching if on Npgsql provider, 
+        // otherwise fallback to string comparison.
+        if (_context.Database.IsNpgsql())
+        {
+            return await _context.Authors
+                .FirstOrDefaultAsync(a => EF.Functions.ILike(a.Name, name), cancellationToken);
+        }
+
+        return await _context.Authors
+            .FirstOrDefaultAsync(
+                a => a.Name.ToLower() == name.ToLower(),
+                cancellationToken);
+    }
+
+    /// <inheritdoc />
+    public void AddAuthor(Author author) => _context.Authors.Add(author);
+
+    /// <inheritdoc />
+    public void AddEbook(Ebook ebook)
+    {
+        // Explicitly set the readiness flag to false upon addition
+        ebook.IsReadyForReading = false;
+        _context.Ebooks.Add(ebook);
+    }
+
+    /// <inheritdoc />
+    public Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
+        => _context.SaveChangesAsync(cancellationToken);
+}