Added worker app.

JSMR.Worker/Services/FileCheckpointStore.cs

@@ -0,0 +1,28 @@
+namespace JSMR.Worker.Services;
+
+public sealed class FileCheckpointStore : ICheckpointStore
+{
+    private readonly string _root = Path.Combine(AppContext.BaseDirectory, "State");
+
+    public FileCheckpointStore() => Directory.CreateDirectory(_root);
+
+    public Task<int?> GetLastPageAsync(string locale, CancellationToken ct)
+    {
+        string path = Path.Combine(_root, $"scan.{locale}.page");
+        if (!File.Exists(path))
+            return Task.FromResult<int?>(null);
+
+        if (int.TryParse(File.ReadAllText(path).Trim(), out var n))
+            return Task.FromResult<int?>(n);
+
+        return Task.FromResult<int?>(null);
+    }
+
+    public Task SaveLastPageAsync(string locale, int page, CancellationToken ct)
+    {
+        string path = Path.Combine(_root, $"scan.{locale}.page");
+        File.WriteAllText(path, page.ToString());
+
+        return Task.CompletedTask;
+    }
+}