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.Infrastructure/DependencyInjection.cs

@@ -1,6 +1,5 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Configuration;
-using Pgvector.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.AI;
 using GeminiDotnet;
@@ -20,6 +19,10 @@ using NexusReader.Domain.Entities;
 using Microsoft.AspNetCore.Identity;
 using Microsoft.AspNetCore.Authorization;
 using NexusReader.Application.Security.Authorization;
+using Qdrant.Client;
+using Neo4j.Driver;
+using Hangfire;
+using Hangfire.PostgreSql;
 
 namespace NexusReader.Infrastructure;
 
@@ -31,12 +34,12 @@ public static class DependencyInjection
         if (!string.IsNullOrEmpty(pgConnectionString))
         {
             services.AddDbContextFactory<AppDbContext>(options =>
-                options.UseNpgsql(pgConnectionString, x => x.UseVector()),
+                options.UseNpgsql(pgConnectionString),
                 ServiceLifetime.Scoped);
 
             // Also register a scoped DbContext for repositories that need it
             services.AddDbContext<AppDbContext>(options =>
-                options.UseNpgsql(pgConnectionString, x => x.UseVector()));
+                options.UseNpgsql(pgConnectionString));
         }
         else
         {
@@ -49,6 +52,23 @@ public static class DependencyInjection
                 options.UseSqlite(sqliteConnectionString));
         }
 
+        // Qdrant Client registration
+        var qdrantUrl = configuration.GetConnectionString("QdrantConnection") ?? "http://localhost:6334";
+        services.AddSingleton<QdrantClient>(sp => new QdrantClient(new Uri(qdrantUrl)));
+
+        // Neo4j Driver registration
+        var neo4jUrl = configuration.GetConnectionString("Neo4jConnection") ?? "bolt://localhost:7687";
+        services.AddSingleton<IDriver>(sp => GraphDatabase.Driver(neo4jUrl, AuthTokens.None));
+
+        // Hangfire registration
+        if (!string.IsNullOrEmpty(pgConnectionString))
+        {
+            services.AddHangfire(config => config
+                .UseRecommendedSerializerSettings()
+                .UsePostgreSqlStorage(options => options.UseNpgsqlConnection(pgConnectionString)));
+            services.AddHangfireServer();
+        }
+
         services.Configure<AiSettings>(configuration.GetSection(AiSettings.SectionName));
         services.Configure<StripeSettings>(configuration.GetSection(StripeSettings.SectionName));
         var aiSettings = configuration.GetSection(AiSettings.SectionName).Get<AiSettings>() ?? new AiSettings();