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/appsettings.json

@@ -1,6 +1,6 @@
 {
-  "ConnectionStrings": {
-    "Default": "Host=localhost;Port=5432;Database=notification_service;Username=postgres;Password=postgres"
+  "App": {
+    "ConnectionString": "Host=localhost;Port=5432;Database=notification_service;Username=postgres;Password=postgres"
   },
   "Serilog": {
     "MinimumLevel": {