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/Repositories/EmailTemplateRepository.cs

@@ -0,0 +1,77 @@
+using HrynCo.NotificationService.DAL.Abstract.Repositories;
+using HrynCo.NotificationService.DAL.Abstract.Templates;
+using HrynCo.NotificationService.DAL.EF.Core;
+using HrynCo.NotificationService.DAL.EF.Entities;
+using Microsoft.EntityFrameworkCore;
+
+namespace HrynCo.NotificationService.DAL.EF.Repositories;
+
+internal sealed class EmailTemplateRepository : EfRepository<EmailTemplateEntity>, IEmailEmailTemplateRepository
+{
+    public EmailTemplateRepository(NotificationDbContext dbContext) : base(dbContext)
+    {
+    }
+
+    public async Task<IReadOnlyList<EmailTemplate>> GetByServiceAsync(string serviceName, CancellationToken ct = default)
+    {
+        List<EmailTemplateEntity> entities = await DbSet
+            .Where(x => x.ServiceName == serviceName)
+            .ToListAsync(ct);
+
+        return entities.Select(MapToDomain).ToList();
+    }
+
+    public async Task<EmailTemplate?> GetAsync(string serviceName, string key, string languageCode, CancellationToken ct = default)
+    {
+        EmailTemplateEntity? entity = await DbSet.FirstOrDefaultAsync(
+            x => x.ServiceName == serviceName && x.Key == key && x.LanguageCode == languageCode, ct);
+
+        return entity is null ? null : MapToDomain(entity);
+    }
+
+    public Task AddAsync(EmailTemplate EmailTemplate, CancellationToken ct = default) =>
+        base.AddAsync(MapToEntity(EmailTemplate), ct);
+
+    public Task UpdateAsync(EmailTemplate EmailTemplate, CancellationToken ct = default)
+    {
+        EmailTemplateEntity entity = MapToEntity(EmailTemplate);
+        entity.Updated = DateTimeOffset.UtcNow;
+        Update(entity);
+        return Task.CompletedTask;
+    }
+
+    public async Task DeleteAsync(EmailTemplate EmailTemplate, CancellationToken ct = default)
+    {
+        EmailTemplateEntity? entity = await DbSet.FindAsync([EmailTemplate.Id], ct);
+        if (entity is not null)
+            Delete(entity);
+    }
+
+    private static EmailTemplate MapToDomain(EmailTemplateEntity e) => new()
+    {
+        Id = e.Id,
+        ServiceName = e.ServiceName,
+        Key = e.Key,
+        LanguageCode = e.LanguageCode,
+        Subject = e.Subject,
+        HtmlBody = e.HtmlBody,
+        TextBody = e.TextBody,
+        Variables = e.Variables.Select(v => new EmailTemplateVariable { Name = v.Name, Required = v.Required }).ToList(),
+        Created = e.Created,
+        Updated = e.Updated
+    };
+
+    private static EmailTemplateEntity MapToEntity(EmailTemplate t) => new()
+    {
+        Id = t.Id,
+        ServiceName = t.ServiceName,
+        Key = t.Key,
+        LanguageCode = t.LanguageCode,
+        Subject = t.Subject,
+        HtmlBody = t.HtmlBody,
+        TextBody = t.TextBody,
+        Variables = t.Variables.Select(v => new EmailTemplateVariableData { Name = v.Name, Required = v.Required }).ToList(),
+        Created = t.Created,
+        Updated = t.Updated
+    };
+}