feat(rag): implement KM-RAG retrieval read-path, API endpoints, global Q&A UI, and unit tests (#49)

This Pull Request implements the complete **Retrieval module (Read Path)** for the Knowledge-Map RAG (KM-RAG) architecture within the NexusReader platform. It resolves all requirements for vector-based semantic search, Neo4j graph context expansion, structured grounding with Google Gemini, API/Wasm integration, and an interactive front-end global Q&A panel.

Resolves #48

### 🚀 Key Implementations

1. **Grounded DTOs & Schema Definition**
   - Added `GroundedResponseDto` and `CitationDto` for strict JSON Schema matching with Gemini.

2. **Core Service & Read Path Logic**
   - Implemented the robust **5-step pipeline** in `KnowledgeService.AskQuestionAsync`:
     1. *Embedding*: Query vectorization using `IEmbeddingGenerator`.
     2. *Semantic Search*: Multi-tenant vector search with Qdrant, supporting scoping to a specific book or global search.
     3. *Graph Expansion*: Fetching connected concepts and parent relationships using Neo4j Cypher.
     4. *Citation Hydration*: Cross-referencing results with PostgreSQL to fetch book titles and accurate chapter citations.
     5. *Grounded Generation*: Strict structured generation via `IChatClient` (Gemini) preventing hallucinations and using citations.

3. **CQRS & Endpoints**
   - Added `AskLibraryQuestionQuery` and its handler.
   - Mapped `/api/knowledge/ask` and `/api/knowledge/search` endpoints inside `Program.cs`.
   - Updated `WasmKnowledgeService` to support proxying retrieval requests.

4. **Premium Blazor UI Panel**
   - Implemented `/intelligence` (Global AI Q&A) with a curated HSL palette, dark theme, smooth micro-animations, loading shimmers, and side-by-side citation cards.
   - Registered the panel within the `MainHubLayout` sidebar.

5. **Test Coverage**
   - Wrote comprehensive xUnit tests in `QueryTests.cs` using Moq and FluentAssertions to assert that handlers correctly validate input and interact with services.

### 🧪 Verification
- Verified compilation and build gate successfully (`dotnet build`: 0 errors).
- All 7 tests passed perfectly (`dotnet test`).

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #49
Reviewed-by: Marek Jaisński <jasins.marek@gmail.com>
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

tests/NexusReader.Application.Tests/Queries/QueryTests.cs

@@ -148,6 +148,57 @@ public class QueryTests : IDisposable
         result.Value.First().ContentHash.Should().Be("hash-123");
     }
 
+    [Fact]
+    public async Task AskLibraryQuestionQuery_WithEmptyQuestion_ReturnsFailure()
+    {
+        // Arrange
+        var knowledgeServiceMock = new Mock<IKnowledgeService>();
+        var handler = new AskLibraryQuestionQueryHandler(knowledgeServiceMock.Object);
+        var query = new AskLibraryQuestionQuery("", "tenant-123");
+
+        // Act
+        var result = await handler.Handle(query, CancellationToken.None);
+
+        // Assert
+        result.IsSuccess.Should().BeFalse();
+        result.Errors.First().Message.Should().Be("Question cannot be empty.");
+    }
+
+    [Fact]
+    public async Task AskLibraryQuestionQuery_WithValidQuestion_CallsKnowledgeService()
+    {
+        // Arrange
+        var knowledgeServiceMock = new Mock<IKnowledgeService>();
+        var expectedResponse = new GroundedResponseDto
+        {
+            Answer = "Based on the book, water boils at 100 degrees Celsius.",
+            Citations = new List<CitationDto>
+            {
+                new CitationDto
+                {
+                    CitationId = "chunk-1",
+                    Snippet = "Water boils at 100 degrees Celsius.",
+                    SourceBook = "Physics 101"
+                }
+            }
+        };
+
+        knowledgeServiceMock.Setup(s => s.AskQuestionAsync("what temp does water boil?", "tenant-123", null, 5, It.IsAny<CancellationToken>()))
+            .ReturnsAsync(Result.Ok(expectedResponse));
+
+        var handler = new AskLibraryQuestionQueryHandler(knowledgeServiceMock.Object);
+        var query = new AskLibraryQuestionQuery("what temp does water boil?", "tenant-123");
+
+        // Act
+        var result = await handler.Handle(query, CancellationToken.None);
+
+        // Assert
+        result.IsSuccess.Should().BeTrue();
+        result.Value.Answer.Should().Be("Based on the book, water boils at 100 degrees Celsius.");
+        result.Value.Citations.Should().HaveCount(1);
+        result.Value.Citations.First().CitationId.Should().Be("chunk-1");
+    }
+
     public void Dispose()
     {
         _connection.Close();