Redo indexing mechanism (#16)

* Bumped up the line length because I fear no man

* Refactored indexing

Previously, indexing worked by collecting the video IDs of only videos
that matched indexing criteria. This new model instead stores ALL videos
for a given source, but will only _download_ videos that meet that criteria.
This lets us backfill without indexing, makes it easier to add in other
backends, lets us download one-off videos for a source that don't quite
meet criteria, you name it.

* Updated media finders to respect format filters; Added credo file

priv/repo/migrations/20240208194729_add_indexing_attributes_to_media_items.exs

@@ -0,0 +1,10 @@
+defmodule Pinchflat.Repo.Migrations.AddIndexingAttributesToMediaItems do
+  use Ecto.Migration
+
+  def change do
+    alter table(:media_items) do
+      add :livestream, :boolean, default: false, null: false
+      add :original_url, :string, null: false
+    end
+  end
+end