[Enhancement] Capture playlist_index for use with output templates (#315)

* Started capturing playlist_index on indexing pass

* Added playlist_index as a media item field

* Added playlist index to output variable templates

* Improved the way playlist_indexes are rejected on update

* Updated docs

* Undid unneeded changes

lib/pinchflat/yt_dlp/media.ex

@@ -22,7 +22,8 @@ defmodule Pinchflat.YtDlp.Media do
     :livestream,
     :short_form_content,
     :uploaded_at,
-    :duration_seconds
+    :duration_seconds,
+    :playlist_index
   ]
 
   alias __MODULE__
@@ -63,7 +64,7 @@ defmodule Pinchflat.YtDlp.Media do
   @doc """
   Returns a map representing the media at the given URL.
 
-  Returns {:ok, [map()]} | {:error, any, ...}.
+  Returns {:ok, %Media{}} | {:error, any, ...}.
   """
   def get_media_attributes(url) do
     runner = Application.get_env(:pinchflat, :yt_dlp_runner)
@@ -84,9 +85,11 @@ defmodule Pinchflat.YtDlp.Media do
 
   @doc """
   Returns the output template for yt-dlp's indexing command.
+
+  NOTE: playlist_index is really only useful for playlists that will never change their order.
   """
   def indexing_output_template do
-    "%(.{id,title,was_live,webpage_url,description,aspect_ratio,duration,upload_date,timestamp})j"
+    "%(.{id,title,was_live,webpage_url,description,aspect_ratio,duration,upload_date,timestamp,playlist_index})j"
   end
 
   @doc """
@@ -104,7 +107,8 @@ defmodule Pinchflat.YtDlp.Media do
       livestream: !!response["was_live"],
       duration_seconds: response["duration"] && round(response["duration"]),
       short_form_content: response["webpage_url"] && short_form_content?(response),
-      uploaded_at: response["upload_date"] && parse_uploaded_at(response)
+      uploaded_at: response["upload_date"] && parse_uploaded_at(response),
+      playlist_index: response["playlist_index"] || 0
     }
   end