Integrate Serilog and add configuration classes

Enhanced logging with Serilog, including request logging and
structured exception handling during startup. Added support
for the Options Pattern with new configuration classes:
`DocumentOperatorSettings`, `RedisSettings`, and
`ApiKeySettings`. Introduced `TenantInfo` class for tenant
management. Updated project files to include new dependencies
and removed unused `Configuration` folder reference.

DocumentOperator.API/Program.cs

@@ -1,25 +1,72 @@
+using Serilog;
+using DocumentOperator.Infrastructure.Configuration;
+
 var builder = WebApplication.CreateBuilder(args);
 
-// Add services to the container.
+// ========================================
+// 1. Serilog Configuration
+// ========================================
+Log.Logger = new LoggerConfiguration()
+    .ReadFrom.Configuration(builder.Configuration)
+    .Enrich.FromLogContext()
+    .Enrich.WithProperty("Application", "DocumentOperator")
+    .CreateLogger();
 
-builder.Services.AddControllers();
-// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
-builder.Services.AddEndpointsApiExplorer();
-builder.Services.AddSwaggerGen();
+builder.Host.UseSerilog();
 
-var app = builder.Build();
+Log.Information("Starting DocumentOperator API...");
 
-// Configure the HTTP request pipeline.
-if (app.Environment.IsDevelopment())
+try
 {
-    app.UseSwagger();
-    app.UseSwaggerUI();
+    // ========================================
+    // 2. Options Pattern Configuration
+    // ========================================
+    builder.Services.Configure<DocumentOperatorSettings>(
+        builder.Configuration.GetSection(DocumentOperatorSettings.SectionName));
+
+    builder.Services.Configure<RedisSettings>(
+        builder.Configuration.GetSection(RedisSettings.SectionName));
+
+    builder.Services.Configure<ApiKeySettings>(
+        builder.Configuration.GetSection(ApiKeySettings.SectionName));
+
+    // ========================================
+    // 3. Services
+    // ========================================
+    builder.Services.AddControllers();
+    builder.Services.AddEndpointsApiExplorer();
+    builder.Services.AddSwaggerGen();
+
+    // ========================================
+    // 4. Build App
+    // ========================================
+    var app = builder.Build();
+
+    // ========================================
+    // 5. Middleware Pipeline
+    // ========================================
+    if (app.Environment.IsDevelopment())
+    {
+        app.UseSwagger();
+        app.UseSwaggerUI();
+    }
+
+    app.UseSerilogRequestLogging(); // Log HTTP Requests
+
+    app.UseHttpsRedirection();
+    app.UseAuthorization();
+    app.MapControllers();
+
+    Log.Information("DocumentOperator API started successfully");
+
+    app.Run();
 }
-
-app.UseHttpsRedirection();
-
-app.UseAuthorization();
-
-app.MapControllers();
-
-app.Run();
+catch (Exception ex)
+{
+    Log.Fatal(ex, "Application startup failed");
+    throw;
+}
+finally
+{
+    Log.CloseAndFlush();
+}