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.Client/Services/WasmKnowledgeService.cs

@@ -73,6 +73,26 @@ public class WasmKnowledgeService : IKnowledgeService
         }
     }
 
+    public async Task<Result<List<SemanticSearchResultDto>>> SearchLibrarySemanticallyAsync(string queryText, string tenantId, int limit, CancellationToken cancellationToken = default)
+    {
+        try
+        {
+            var response = await _httpClient.PostAsJsonAsync("/api/knowledge/search", new { queryText, tenantId, limit }, cancellationToken);
+            if (response.IsSuccessStatusCode)
+            {
+                var searchResults = await response.Content.ReadFromJsonAsync<List<SemanticSearchResultDto>>(cancellationToken: cancellationToken);
+                return searchResults != null ? Result.Ok(searchResults) : Result.Ok(new List<SemanticSearchResultDto>());
+            }
+
+            var errorBody = await response.Content.ReadAsStringAsync(cancellationToken);
+            return Result.Fail($"Server error ({response.StatusCode}): {errorBody}");
+        }
+        catch (Exception ex)
+        {
+            return Result.Fail(new Error($"Network error: {ex.Message}").CausedBy(ex));
+        }
+    }
+
     private async Task<Result<KnowledgePacket>> CallKnowledgeApiAsync(string endpoint, string text, Guid? ebookId, CancellationToken cancellationToken)
     {
         try