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.Web/Views/AdminServices/DeleteLanguage.cshtml

@@ -0,0 +1,13 @@
+@using HrynCo.NotificationService.Web.Controllers.Admin.ViewModels
+@model ServiceLanguageDeleteViewModel
+@{ ViewData["Title"] = "Delete service language and versions"; }
+<h2>Delete language @Model.Code from @Model.ServiceName?</h2>
+<div asp-validation-summary="All" class="text-danger" role="alert"></div>
+<div class="alert alert-danger">This permanently deletes <strong>@Model.VersionCount template version(s)</strong> using this language in this service and removes the language code. Template definitions, their keys and shared variables are retained.</div>
+<form method="post" action="/admin/services/@Model.ServiceId/languages/@Model.LanguageId/delete">
+    @Html.AntiForgeryToken()
+    <input asp-for="VersionCount" type="hidden" /><input asp-for="Revision" type="hidden" />
+    <div class="form-check mb-3"><input asp-for="ConfirmDelete" class="form-check-input" /><label asp-for="ConfirmDelete" class="form-check-label">I confirm deletion of @Model.VersionCount version(s) and language @Model.Code.</label></div>
+    <button type="submit" class="btn btn-danger">Delete @Model.VersionCount version(s) and language</button>
+    <a class="btn btn-secondary" href="/admin/services/@Model.ServiceId/edit">Cancel</a>
+</form>