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/Controllers/EmailChannels/CreateEmailChannelRequest.cs

@@ -0,0 +1,14 @@
+using HrynCo.NotificationService.DAL.Abstract.Providers;
+
+namespace HrynCo.NotificationService.Api.Controllers.EmailChannels;
+
+public sealed record CreateEmailChannelRequest(
+    string ServiceName,
+    int Priority,
+    EmailChannelType ChannelType,
+    EmailChannelSettings Settings,
+    int? DailyLimit,
+    int? MonthlyLimit,
+    int WarnThresholdPercent,
+    bool IsActive
+);