feat(recommendations): implement contextual recommendation engine (#76)

Resolves #75

### Description
This pull request implements a smart, Native AOT-compliant contextual recommendation engine for the desktop dashboard to drive user retention and cross-book monetization.

### Key Changes
1. **Application Layer**:
   - Declared `IUserReadingStateStore` interface to decouple reading state discovery.
   - Added `IVectorSearchStore.SearchGlobalExcludeAsync(...)` to abstract semantic similarity searches with exclusions.
   - Defined `GetContextualRecommendationsQuery` and response DTOs (`ContextualRecommendationResponse`, `RecommendationDto`).
2. **Infrastructure Layer**:
   - Implemented `UserReadingStateStore` using EF Core with DbContext pooling.
   - Implemented `SearchGlobalExcludeAsync` in `VectorSearchStore` to construct gRPC Qdrant filters (excluding the active book ID) and fetch `bookTitle` and `chapterTitle` from point payloads.
   - Implemented `GetContextualRecommendationsQueryHandler` using clean abstractions.
3. **Web & Serialization Layer**:
   - Mapped `GET /api/recommendations` endpoint.
   - Registered types in `AppJsonContext.cs` for AOT-compliant JSON serialization.
4. **Verification**:
   - Added complete unit test coverage in `GetContextualRecommendationsQueryTests.cs`. All 30 unit tests pass.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #76
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.Application/Abstractions/Persistence/IUserLibraryStore.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace NexusReader.Application.Abstractions.Persistence;
+
+/// <summary>
+/// Provides access to user library ownership details, decoupling the relational database
+/// structures from vector search and intelligence query operations.
+/// </summary>
+public interface IUserLibraryStore
+{
+    /// <summary>
+    /// Retrieves a list of book IDs that are owned by or uploaded for the specified user.
+    /// </summary>
+    Task<List<Guid>> GetOwnedBookIdsAsync(string userId, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Retrieves a dictionary mapping book IDs to their titles.
+    /// </summary>
+    Task<Dictionary<Guid, string>> GetBookTitlesAsync(List<Guid> bookIds, CancellationToken cancellationToken = default);
+}