feat: KM-RAG Polyglot Ingestion Pipeline Migration (#46)

Resolves the KM-RAG Polyglot Persistence and Background Ingestion Pipeline Migration task.

### Key Changes
1. **Infrastructure Migration**: Integrated Qdrant (for vector embeddings) and Neo4j (for concept graphs), reducing reliance on PostgreSQL pgvector storage.
2. **Concurrent Background Job**: Implemented a robust Hangfire `EbookIngestionJob` utilizing Polly exponential retries for transient 429 rate limits, executing three core ingestion tasks concurrently via `Task.WhenAll`.
3. **Data Layer**: Standardized database schemas and entities; retained `Pgvector.EntityFrameworkCore` for migration compilation compatibility.
4. **Wasm Client & Tests**: Implemented client support for semantic search and refactored related tests in `QueryTests.cs` to mock `IKnowledgeService`.

### Verification Status
- **Build**: Successfully compiles with `dotnet build NexusReader.slnx --no-restore` (0 errors).
- **Tests**: All 5 unit tests pass cleanly with `dotnet test NexusReader.slnx --no-restore`.

**Resolve** #47

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #46
Reviewed-by: Marek Jaisński <jasins.marek@gmail.com>
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.Web/NexusReader.Web.csproj

@@ -19,6 +19,7 @@
     <PackageReference Include="VersOne.Epub" Version="3.3.6" />
     <ProjectReference Include="..\NexusReader.Web.Client\NexusReader.Web.Client.csproj" />
     <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly.Server" Version="10.0.7" />
+    <PackageReference Include="Hangfire.AspNetCore" Version="1.8.23" />
   </ItemGroup>
 
   <ItemGroup>

src/NexusReader.Web/Program.cs

@@ -24,6 +24,7 @@ using Stripe;
 using Microsoft.Extensions.AI;
 using NexusReader.Application.Abstractions.Persistence;
 using NexusReader.Application.Abstractions.Messaging;
+using Hangfire;
 
 AppContext.SetSwitch("Npgsql.EnableLegacyTimestampBehavior", true);
 
@@ -159,6 +160,8 @@ builder.Services.Configure<IdentityOptions>(options =>
 
 var app = builder.Build();
 
+app.UseHangfireDashboard();
+
 // Startup Validation
 using (var scope = app.Services.CreateScope())
 {