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/Repositories/IEmailChannelRepository.cs

@@ -0,0 +1,12 @@
+using HrynCo.NotificationService.DAL.Abstract.Providers;
+
+namespace HrynCo.NotificationService.DAL.Abstract.Repositories;
+
+public interface IEmailChannelRepository
+{
+    Task<IReadOnlyList<EmailChannel>> GetByServiceAsync(string serviceName, CancellationToken ct = default);
+    Task<EmailChannel?> GetByIdAsync(Guid id, CancellationToken ct = default);
+    Task AddAsync(EmailChannel channel, CancellationToken ct = default);
+    Task UpdateAsync(EmailChannel channel, CancellationToken ct = default);
+    Task DeleteAsync(EmailChannel channel, CancellationToken ct = default);
+}

HrynCo.NotificationService.DAL.Abstract/Repositories/IEmailChannelUsageRepository.cs

@@ -2,9 +2,9 @@ using HrynCo.NotificationService.DAL.Abstract.Providers;
 
 namespace HrynCo.NotificationService.DAL.Abstract.Repositories;
 
-public interface IProviderUsageRepository
+public interface IEmailChannelUsageRepository
 {
     Task<int> GetDailyCountAsync(Guid providerId, DateOnly date, CancellationToken ct = default);
     Task<int> GetMonthlyCountAsync(Guid providerId, int year, int month, CancellationToken ct = default);
-    Task IncrementAsync(Guid providerId, DateOnly date, CancellationToken ct = default);
+    Task IncrementUsageAsync(Guid providerId, DateOnly date, CancellationToken ct = default);
 }

HrynCo.NotificationService.DAL.Abstract/Repositories/IEmailTemplateRepository.cs

@@ -0,0 +1,12 @@
+using HrynCo.NotificationService.DAL.Abstract.Templates;
+
+namespace HrynCo.NotificationService.DAL.Abstract.Repositories;
+
+public interface IEmailEmailTemplateRepository
+{
+    Task<IReadOnlyList<EmailTemplate>> GetByServiceAsync(string serviceName, CancellationToken ct = default);
+    Task<EmailTemplate?> GetAsync(string serviceName, string key, string languageCode, CancellationToken ct = default);
+    Task AddAsync(EmailTemplate EmailTemplate, CancellationToken ct = default);
+    Task UpdateAsync(EmailTemplate EmailTemplate, CancellationToken ct = default);
+    Task DeleteAsync(EmailTemplate EmailTemplate, CancellationToken ct = default);
+}

HrynCo.NotificationService.DAL.Abstract/Repositories/IProviderRepository.cs

@@ -1,12 +0,0 @@
-using HrynCo.NotificationService.DAL.Abstract.Providers;
-
-namespace HrynCo.NotificationService.DAL.Abstract.Repositories;
-
-public interface IProviderRepository
-{
-    Task<IReadOnlyList<Provider>> GetByServiceAsync(string serviceName, CancellationToken ct = default);
-    Task<Provider?> GetByIdAsync(Guid id, CancellationToken ct = default);
-    Task AddAsync(Provider provider, CancellationToken ct = default);
-    Task UpdateAsync(Provider provider, CancellationToken ct = default);
-    Task DeleteAsync(Provider provider, CancellationToken ct = default);
-}

HrynCo.NotificationService.DAL.Abstract/Repositories/ITemplateRepository.cs

@@ -1,12 +0,0 @@
-using HrynCo.NotificationService.DAL.Abstract.Templates;
-
-namespace HrynCo.NotificationService.DAL.Abstract.Repositories;
-
-public interface ITemplateRepository
-{
-    Task<IReadOnlyList<Template>> GetByServiceAsync(string serviceName, CancellationToken ct = default);
-    Task<Template?> GetAsync(string serviceName, string key, string languageCode, CancellationToken ct = default);
-    Task AddAsync(Template template, CancellationToken ct = default);
-    Task UpdateAsync(Template template, CancellationToken ct = default);
-    Task DeleteAsync(Template template, CancellationToken ct = default);
-}