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/EmailTemplateVariableData.cs

@@ -0,0 +1,7 @@
+namespace HrynCo.NotificationService.DAL.EF.Entities;
+
+internal sealed class EmailTemplateVariableData
+{
+    public required string Name { get; set; }
+    public bool Required { get; set; }
+}