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.Worker/AppSettings.cs

@@ -0,0 +1,8 @@
+namespace HrynCo.NotificationService.Worker;
+
+public sealed class AppSettings
+{
+    public const string SectionName = "App";
+
+    public string ConnectionString { get; init; } = string.Empty;
+}