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/Services/BookStorageService.cs

@@ -35,7 +35,9 @@ public class BookStorageService : IBookStorageService
             await data.CopyToAsync(fileStream);
         }
         
-        return Path.Combine("uploads", uniqueFileName);
+        // Use forward-slash explicitly: Path.Combine produces backslashes on Windows
+        // which would cause 404s when stored as web-relative paths.
+        return $"uploads/{uniqueFileName}";
     }
 
     public async Task<string?> SaveCoverAsync(byte[] data, string fileName)
@@ -58,7 +60,7 @@ public class BookStorageService : IBookStorageService
             await data.CopyToAsync(fileStream);
         }
         
-        return Path.Combine("covers", uniqueFileName);
+        return $"covers/{uniqueFileName}";
     }
 
     private void EnsureDirectoryExists(string path)