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/EmailChannel.cs

@@ -2,14 +2,14 @@ using HrynCo.NotificationService.DAL.Abstract.Entities;
 
 namespace HrynCo.NotificationService.DAL.Abstract.Providers;
 
-public class Provider : Entity
+public class EmailChannel : Entity
 {
     public required string ServiceName { get; set; }
     public int Priority { get; set; }
-    public ProviderType ProviderType { get; set; }
-    public required ProviderSettings Settings { get; set; }
+    public EmailChannelType EmailChannelType { get; set; }
+    public required EmailChannelSettings Settings { get; set; }
     public int? DailyLimit { get; set; }
     public int? MonthlyLimit { get; set; }
     public int WarnThresholdPercent { get; set; } = 90;
     public bool IsActive { get; set; } = true;
-}
+}

HrynCo.NotificationService.DAL.Abstract/Providers/EmailChannelSettings.cs

@@ -1,13 +1,13 @@
 namespace HrynCo.NotificationService.DAL.Abstract.Providers;
 
-public abstract class ProviderSettings
+public abstract class EmailChannelSettings
 {
-    public abstract ProviderType ProviderType { get; }
+    public abstract EmailChannelType EmailChannelType { get; }
 }
 
-public class SmtpProviderSettings : ProviderSettings
+public class SmtpChannelSettings : EmailChannelSettings
 {
-    public override ProviderType ProviderType => ProviderType.Smtp;
+    public override EmailChannelType EmailChannelType => EmailChannelType.Smtp;
 
     public required string Host { get; set; }
     public int Port { get; set; } = 587;

HrynCo.NotificationService.DAL.Abstract/Providers/EmailChannelType.cs

@@ -1,6 +1,7 @@
 namespace HrynCo.NotificationService.DAL.Abstract.Providers;
 
-public enum ProviderType
+public enum EmailChannelType
 {
+    Undefined = 0,
     Smtp = 1
 }

HrynCo.NotificationService.DAL.Abstract/Providers/EmailChannelUsage.cs

@@ -3,10 +3,10 @@ using HrynCo.NotificationService.DAL.Abstract.Entities;
 namespace HrynCo.NotificationService.DAL.Abstract.Providers;
 
 /// <summary>
-/// Tracks email send counts per provider per calendar day.
+/// Tracks email send counts per EmailChannel per calendar day.
 /// Monthly counts are derived by summing daily records within a month.
 /// </summary>
-public class ProviderUsage : Entity
+public class EmailChannelUsage : Entity
 {
     public Guid ProviderId { get; set; }
     public DateOnly Date { get; set; }