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

@@ -2,7 +2,7 @@ using HrynCo.NotificationService.DAL.Abstract.Entities;
 
 namespace HrynCo.NotificationService.DAL.Abstract.Templates;
 
-public class Template : Entity
+public class EmailTemplate : Entity
 {
     public required string ServiceName { get; set; }
     public required string Key { get; set; }
@@ -10,5 +10,5 @@ public class Template : Entity
     public required string Subject { get; set; }
     public required string HtmlBody { get; set; }
     public required string TextBody { get; set; }
-    public IReadOnlyList<TemplateVariable> Variables { get; set; } = [];
+    public IReadOnlyList<EmailTemplateVariable> Variables { get; set; } = [];
 }

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

@@ -1,6 +1,6 @@
 namespace HrynCo.NotificationService.DAL.Abstract.Templates;
 
-public record TemplateVariable
+public record EmailTemplateVariable
 {
     public required string Name { get; init; }
     public bool Required { get; init; }