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/Providers/EmailChannelSettings.cs

@@ -0,0 +1,21 @@
+namespace HrynCo.NotificationService.DAL.Abstract.Providers;
+
+public abstract class EmailChannelSettings
+{
+    public abstract EmailChannelType EmailChannelType { get; }
+}
+
+public class SmtpChannelSettings : EmailChannelSettings
+{
+    public override EmailChannelType EmailChannelType => EmailChannelType.Smtp;
+
+    public required string Host { get; set; }
+    public int Port { get; set; } = 587;
+    public required string Username { get; set; }
+    public required string Password { get; set; }
+    public bool UseSsl { get; set; } = true;
+    public required string FromEmail { get; set; }
+    public required string FromName { get; set; }
+    public required string AppDisplayName { get; set; }
+    public required string AppBaseUrl { get; set; }
+}