feat: introduce AppSettings per application

- AppSettings class in Api and Worker with SectionName constant
- appsettings.json: replaced ConnectionStrings section with App section
- Program.cs: bind AppSettings at startup, register as singleton
- Connection string now sourced from AppSettings.ConnectionString

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Api/Program.cs

@@ -1,3 +1,4 @@
+using HrynCo.NotificationService.Api;
 using HrynCo.NotificationService.DAL.EF;
 using HrynCo.NotificationService.Services;
 using Serilog;
@@ -7,13 +8,14 @@ var builder = WebApplication.CreateBuilder(args);
 builder.Host.UseSerilog((context, lc) =>
     lc.ReadFrom.Configuration(context.Configuration));
 
+var appSettings = builder.Configuration
+    .GetSection(AppSettings.SectionName)
+    .Get<AppSettings>() ?? throw new InvalidOperationException("App settings are not configured.");
+
+builder.Services.AddSingleton(appSettings);
 builder.Services.AddOpenApi();
 builder.Services.AddControllers();
-
-string connectionString = builder.Configuration.GetConnectionString("Default")
-    ?? throw new InvalidOperationException("Connection string 'Default' is not configured.");
-
-builder.Services.AddNotificationDataAccess(connectionString);
+builder.Services.AddNotificationDataAccess(appSettings.ConnectionString);
 builder.Services.AddNotificationServices();
 
 var app = builder.Build();