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/Program.cs

@@ -20,6 +20,9 @@ using Microsoft.AspNetCore.Authentication;
 using System.Security.Claims;
 using NexusReader.Infrastructure.Services;
 using Stripe;
+using Microsoft.Extensions.AI;
+using NexusReader.Application.Abstractions.Persistence;
+using NexusReader.Application.Abstractions.Messaging;
 
 AppContext.SetSwitch("Npgsql.EnableLegacyTimestampBehavior", true);
 
@@ -239,10 +242,10 @@ app.MapStaticAssets();
 app.MapHub<NexusReader.Infrastructure.RealTime.SyncHub>("/synchub");
 
 // API endpoint for WASM client to fetch EPUB content
-app.MapGet("/api/epub/{index}", async (int index, IEpubReader epubService, ClaimsPrincipal user) =>
+app.MapGet("/api/epub/{ebookId:guid}/{index:int}", async (Guid ebookId, int index, IEpubReader epubService, ClaimsPrincipal user) =>
 {
     var userId = user.FindFirstValue(ClaimTypes.NameIdentifier);
-    var result = await epubService.GetEpubContentAsync(index, userId);
+    var result = await epubService.GetEpubContentAsync(ebookId, index, userId);
     
     if (result.IsSuccess) return Results.Ok(result.Value);