refactor: rename domain types and introduce TransactionBehavior pattern

- Rename Template -> EmailTemplate, Provider -> EmailChannel,
  ProviderSettings -> EmailChannelSettings, ProviderType -> EmailChannelType,
  ProviderUsage -> EmailChannelUsage throughout all layers
- Add Undefined = 0 to EmailChannelType enum for safe default handling
- Remove SaveChangesAsync from EfRepository methods — repositories now only stage changes
- Add SaveChangesAsync to IUnitOfWork and EfUnitOfWork
- Add TransactionBehavior MediatR pipeline: wraps every handler in a transaction,
  saves and commits on success, rolls back on exception
- Add MediatR package reference to Services project

Ref: IT-628

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

HrynCo.NotificationService.DAL.Abstract/Templates/EmailTemplate.cs

@@ -0,0 +1,14 @@
+using HrynCo.NotificationService.DAL.Abstract.Entities;
+
+namespace HrynCo.NotificationService.DAL.Abstract.Templates;
+
+public class EmailTemplate : Entity
+{
+    public required string ServiceName { get; set; }
+    public required string Key { get; set; }
+    public required string LanguageCode { get; set; }
+    public required string Subject { get; set; }
+    public required string HtmlBody { get; set; }
+    public required string TextBody { get; set; }
+    public IReadOnlyList<EmailTemplateVariable> Variables { get; set; } = [];
+}