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.EF/Entities/EmailChannelEntity.cs

@@ -3,15 +3,15 @@ using HrynCo.NotificationService.DAL.Abstract.Providers;
 
 namespace HrynCo.NotificationService.DAL.EF.Entities;
 
-internal class ProviderEntity : Entity
+internal class EmailChannelEntity : Entity
 {
     public required string ServiceName { get; set; }
     public int Priority { get; set; }
-    public ProviderType ProviderType { get; set; }
+    public EmailChannelType EmailChannelType { get; set; }
 
     /// <summary>
-    /// Provider-specific credentials and settings stored as JSONB.
-    /// Deserialized based on <see cref="ProviderType"/> in the repository.
+    /// EmailChannel-specific credentials and settings stored as JSONB.
+    /// Deserialized based on <see cref="EmailChannelType"/> in the repository.
     /// </summary>
     public required string SettingsJson { get; set; }
 

HrynCo.NotificationService.DAL.EF/Entities/EmailChannelUsageEntity.cs

@@ -2,7 +2,7 @@ using HrynCo.NotificationService.DAL.Abstract.Entities;
 
 namespace HrynCo.NotificationService.DAL.EF.Entities;
 
-internal class ProviderUsageEntity : Entity
+internal class EmailChannelUsageEntity : Entity
 {
     public Guid ProviderId { get; set; }
     public DateOnly Date { get; set; }

HrynCo.NotificationService.DAL.EF/Entities/EmailTemplateEntity.cs

@@ -2,7 +2,7 @@ using HrynCo.NotificationService.DAL.Abstract.Entities;
 
 namespace HrynCo.NotificationService.DAL.EF.Entities;
 
-internal class TemplateEntity : Entity
+internal class EmailTemplateEntity : Entity
 {
     public required string ServiceName { get; set; }
     public required string Key { get; set; }
@@ -10,10 +10,10 @@ internal class TemplateEntity : Entity
     public required string Subject { get; set; }
     public required string HtmlBody { get; set; }
     public required string TextBody { get; set; }
-    public List<TemplateVariableData> Variables { get; set; } = [];
+    public List<EmailTemplateVariableData> Variables { get; set; } = [];
 }
 
-internal class TemplateVariableData
+internal class EmailTemplateVariableData
 {
     public required string Name { get; set; }
     public bool Required { get; set; }