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/Messaging/ISyncBroadcaster.cs

@@ -0,0 +1,38 @@
+namespace NexusReader.Application.Abstractions.Messaging;
+
+/// <summary>
+/// Abstraction for broadcasting real-time sync events to connected clients.
+/// Defined in Application to prevent a direct dependency on SignalR in Application layer handlers.
+/// </summary>
+public interface ISyncBroadcaster
+{
+    /// <summary>
+    /// Broadcasts a reading progress update to all devices belonging to the specified user,
+    /// optionally excluding the originating connection.
+    /// </summary>
+    /// <param name="userId">The user whose other devices should be notified.</param>
+    /// <param name="pageId">The block/page ID the user has reached.</param>
+    /// <param name="timestamp">The server-side UTC timestamp of the update.</param>
+    /// <param name="excludedConnectionId">SignalR connection ID to exclude (the sender's device).</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    Task BroadcastProgressAsync(
+        string userId,
+        string pageId,
+        DateTime timestamp,
+        string? excludedConnectionId,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Broadcasts real-time ingestion status updates to a specific user.
+    /// This is used by background workers to provide feedback during AI-intensive processing.
+    /// </summary>
+    /// <param name="userId">The ID of the user who owns the ingestion request.</param>
+    /// <param name="message">A human-readable status message (e.g., "Parsing chapters...").</param>
+    /// <param name="progress">Progress percentage (0.0 to 1.0).</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    Task BroadcastIngestionProgressAsync(
+        string userId,
+        string message,
+        double progress,
+        CancellationToken cancellationToken = default);
+}