Add parallel export with configurable concurrency (default: 8)

Use Task.async_stream for parallel org->md export.
Configurable via EXPORT_CONCURRENCY env var or :export_concurrency config.

lib/org_garden/export.ex

@@ -71,9 +71,15 @@ defmodule OrgGarden.Export do
     e -> {:error, e}
   end
 
+  @default_max_concurrency 8
+
   @doc """
   Export all `.org` files found under `notes_dir`.
 
+  Exports files in parallel for improved performance. The concurrency level
+  can be configured via the `:export_concurrency` application config or
+  the `EXPORT_CONCURRENCY` environment variable. Defaults to #{@default_max_concurrency}.
+
   Returns `{:ok, count}` where `count` is the number of successfully
   exported files, or `{:error, failures}` if any files failed.
   """
@@ -87,13 +93,21 @@ defmodule OrgGarden.Export do
       Logger.warning("No .org files found in #{notes_dir}")
       {:ok, 0}
     else
-      Logger.info("Exporting #{length(org_files)} org file(s) from #{notes_dir}")
+      max_concurrency = get_concurrency()
+      Logger.info("Exporting #{length(org_files)} org file(s) from #{notes_dir} (concurrency: #{max_concurrency})")
 
       results =
-        Enum.map(org_files, fn orgfile ->
-          IO.puts("  exporting: #{orgfile}")
-          {orgfile, export_file(orgfile, notes_dir, output_dir)}
-        end)
+        org_files
+        |> Task.async_stream(
+          fn orgfile ->
+            Logger.info("  exporting: #{orgfile}")
+            {orgfile, export_file(orgfile, notes_dir, output_dir)}
+          end,
+          max_concurrency: max_concurrency,
+          timeout: :infinity,
+          ordered: false
+        )
+        |> Enum.map(fn {:ok, result} -> result end)
 
       failures =
         Enum.filter(results, fn
@@ -109,6 +123,10 @@ defmodule OrgGarden.Export do
     end
   end
 
+  defp get_concurrency do
+    Application.get_env(:org_garden, :export_concurrency, @default_max_concurrency)
+  end
+
   @doc """
   Compute the expected `.md` path for a given `.org` file.