fix(backup): use active SQLite database and validate safe restore (U01)

src/Duempelkas.Infrastructure/Services/DatabaseLease.cs

@@ -0,0 +1,12 @@
+namespace Duempelkas.Infrastructure.Services;
+
+internal sealed class DatabaseLease(SemaphoreSlim? gate) : IDisposable
+{
+    public static async Task<DatabaseLease> AcquireAsync(DatabaseLocation? location)
+    {
+        if (location != null) await location.Writes.WaitAsync();
+        return new DatabaseLease(location?.Writes);
+    }
+
+    public void Dispose() => gate?.Release();
+}