feat: add API controllers for template and channel management

- ApiResponse<T>, ApiError in Api/Infrastructure
- ApiControllerBase with IMediator, FromServiceResult, MapServiceError
- EmailTemplatesController: GET list, GET one, POST, PUT, DELETE
- EmailChannelsController: GET list, GET one, POST, PUT, DELETE

Ref: IT-628

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

HrynCo.NotificationService.Api/Infrastructure/ApiResponse.cs

@@ -0,0 +1,14 @@
+namespace HrynCo.NotificationService.Api.Infrastructure;
+
+public sealed class ApiResponse<T>
+{
+    public T? Data { get; init; } = default;
+    public ApiError? Error { get; init; }
+    public bool Success { get; init; }
+}
+
+public sealed class ApiError
+{
+    public required string Code { get; init; }
+    public required string Message { get; init; }
+}