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/Configurations/EmailChannelEntityConfiguration.cs

@@ -4,9 +4,9 @@ using Microsoft.EntityFrameworkCore.Metadata.Builders;
 
 namespace HrynCo.NotificationService.DAL.EF.Configurations;
 
-internal class ProviderEntityConfiguration : IEntityTypeConfiguration<ProviderEntity>
+internal class EmailChannelEntityConfiguration : IEntityTypeConfiguration<EmailChannelEntity>
 {
-    public void Configure(EntityTypeBuilder<ProviderEntity> builder)
+    public void Configure(EntityTypeBuilder<EmailChannelEntity> builder)
     {
         builder.ToTable("providers");
 
@@ -22,7 +22,7 @@ internal class ProviderEntityConfiguration : IEntityTypeConfiguration<ProviderEn
 
         builder.Property(x => x.Priority).HasColumnName("priority");
 
-        builder.Property(x => x.ProviderType).HasColumnName("provider_type");
+        builder.Property(x => x.EmailChannelType).HasColumnName("provider_type");
 
         builder.Property(x => x.SettingsJson)
             .HasColumnName("settings")

HrynCo.NotificationService.DAL.EF/Configurations/EmailChannelUsageEntityConfiguration.cs

@@ -4,9 +4,9 @@ using Microsoft.EntityFrameworkCore.Metadata.Builders;
 
 namespace HrynCo.NotificationService.DAL.EF.Configurations;
 
-internal class ProviderUsageEntityConfiguration : IEntityTypeConfiguration<ProviderUsageEntity>
+internal class EmailChannelUsageEntityConfiguration : IEntityTypeConfiguration<EmailChannelUsageEntity>
 {
-    public void Configure(EntityTypeBuilder<ProviderUsageEntity> builder)
+    public void Configure(EntityTypeBuilder<EmailChannelUsageEntity> builder)
     {
         builder.ToTable("provider_usage");
 

HrynCo.NotificationService.DAL.EF/Configurations/EmailTemplateEntityConfiguration.cs

@@ -4,9 +4,9 @@ using Microsoft.EntityFrameworkCore.Metadata.Builders;
 
 namespace HrynCo.NotificationService.DAL.EF.Configurations;
 
-internal class TemplateEntityConfiguration : IEntityTypeConfiguration<TemplateEntity>
+internal class EmailEmailTemplateEntityConfiguration : IEntityTypeConfiguration<EmailTemplateEntity>
 {
-    public void Configure(EntityTypeBuilder<TemplateEntity> builder)
+    public void Configure(EntityTypeBuilder<EmailTemplateEntity> builder)
     {
         builder.ToTable("templates");