feat(templates): add service language template management

Add normalized service and template catalogs, scoped administration, migration, validation, and regression coverage.

Ref: IT-1601

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

@@ -4,17 +4,12 @@ namespace HrynCo.NotificationService.DAL.EF.Entities;
 
 internal class EmailTemplateEntity : Entity
 {
-    public required string ServiceName { get; set; }
-    public required string Key { get; set; }
-    public required string LanguageCode { get; set; }
+    public Guid DefinitionId { get; set; }
+    public EmailTemplateDefinitionEntity Definition { get; set; } = null!;
+    public Guid ServiceId { get; set; }
+    public Guid ServiceLanguageId { get; set; }
+    public ServiceLanguageEntity ServiceLanguage { get; set; } = null!;
     public required string Subject { get; set; }
     public required string HtmlBody { get; set; }
     public required string TextBody { get; set; }
-    public List<EmailTemplateVariableData> Variables { get; set; } = [];
-}
-
-internal class EmailTemplateVariableData
-{
-    public required string Name { get; set; }
-    public bool Required { get; set; }
 }