feat(notification): add admin rest management and deletion

Add revision-safe service and template APIs, channel mapping, and validated admin controls.

Ref: IT-1601

HrynCo.NotificationService.DAL.EF/Repositories/EmailChannelRepository.cs

@@ -54,7 +54,7 @@ internal sealed class EmailChannelRepository : NotificationBaseRepository<EmailC
 
     public async Task<EmailChannel?> GetByIdAsync(Guid id, CancellationToken ct = default)
     {
-        EmailChannelEntity? entity = await EfRepository.GetByIdAsync(id);
+        EmailChannelEntity? entity = await EfRepository.Get().AsNoTracking().SingleOrDefaultAsync(c => c.Id == id, ct);
         return entity is null ? null : MapToDomain(entity);
     }