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.Web.IntegrationTests/TemplateCatalogPresentationTests.cs

@@ -58,7 +58,7 @@ public sealed class TemplateCatalogPresentationTests(TemplateDatabaseFixture fix
         var miss = await client.GetStringAsync(baseUrl + "?key=Absent");
         Assert.Matches("(?s)<th[^>]*>\\s*<form[^>]*method=\"get\"[^>]*class=\"template-key-filter\"", miss);
         Assert.Contains("value=\"Absent\"", miss);
-        Assert.Contains("No email templates found", miss);
+        Assert.Contains("No templates match this key.", miss);
         Assert.Contains("class=\"template-actions\">Action</th>", miss);
         Assert.Contains($"href=\"{baseUrl}\" class=\"btn btn-outline-secondary btn-sm\">Clear</a>", miss);
         var filtered = await client.GetStringAsync(baseUrl + "?key=Welcome");
@@ -85,13 +85,13 @@ public sealed class TemplateCatalogPresentationTests(TemplateDatabaseFixture fix
         var empty = await client.GetStringAsync(baseUrl);
         Assert.Contains("No languages configured for this service.", empty);
         Assert.Contains("template-key-filter", empty);
-        Assert.Contains("No email templates found", empty);
+        Assert.Contains("No email templates for this service yet.", empty);
         await scope.ServiceProvider.GetRequiredService<IEmailTemplateDefinitionRepository>()
             .SaveDefinitionAsync(new EmailTemplateDefinition { ServiceName = service.Name, Key = "Empty" }, true);
         var withDefinition = await client.GetStringAsync(baseUrl);
         Assert.Contains("No configured languages", TemplateRow(withDefinition, "Empty"));
         Assert.DoesNotContain("data-state=", withDefinition);
-        Assert.DoesNotContain("No email templates found", withDefinition);
+        Assert.DoesNotContain("No templates match this key.", withDefinition);
     }
 
     private static string TemplateRow(string html, string key)