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/TemplateMutationRunner.cs

@@ -0,0 +1,19 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace HrynCo.NotificationService.DAL.EF;
+
+internal sealed class TemplateMutationRunner(NotificationDbContext db)
+{
+    public async Task ExecuteAsync(Func<Task> write, CancellationToken ct)
+    {
+        await using var transaction = db.Database.CurrentTransaction is null
+            ? await db.Database.BeginTransactionAsync(ct) : null;
+        // Service/language administration and template writes share the same transaction lock.
+        // Counts, catalog membership and revision checks are therefore valid until commit.
+        await db.Database.ExecuteSqlRawAsync("SELECT pg_advisory_xact_lock(1601)", ct);
+        await write();
+        await db.SaveChangesAsync(ct);
+        if (transaction is not null)
+            await transaction.CommitAsync(ct);
+    }
+}