feat: implement AI-driven knowledge extraction service with semantic caching and persistent storage

src/NexusReader.Infrastructure/DependencyInjection.cs

@@ -1,4 +1,10 @@
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Configuration;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.AI;
+using GeminiDotnet;
+using GeminiDotnet.Extensions.AI;
+using NexusReader.Infrastructure.Persistence;
 using NexusReader.Application.Abstractions.Services;
 using NexusReader.Infrastructure.Services;
 
@@ -6,8 +12,26 @@ namespace NexusReader.Infrastructure;
 
 public static class DependencyInjection
 {
-    public static IServiceCollection AddInfrastructure(this IServiceCollection services)
+    public static IServiceCollection AddInfrastructure(this IServiceCollection services, IConfiguration configuration)
     {
+        var connectionString = configuration.GetConnectionString("SqliteConnection") ?? "Data Source=nexus.db";
+        services.AddDbContext<AppDbContext>(options =>
+            options.UseSqlite(connectionString));
+
+        var apiKey = configuration["Ai:Google:ApiKey"];
+        if (string.IsNullOrWhiteSpace(apiKey))
+        {
+            throw new InvalidOperationException("AI Studio ApiKey is missing in configuration (Ai:Google:ApiKey).");
+        }
+        var modelId = configuration["Ai:Google:Model"] ?? "gemini-1.5-flash";
+
+        services.AddSingleton<IChatClient>(new GeminiChatClient(new GeminiClientOptions 
+        { 
+            ApiKey = apiKey, 
+            ModelId = modelId 
+        }));
+
+        services.AddScoped<IKnowledgeService, KnowledgeService>();
         services.AddTransient<IAiGenerateQuizService, FakeAiGenerateQuizService>();
         services.AddTransient<IEpubService, EpubService>();
         return services;