[Performance] Asyncronously delete sources and media profiles (#277)

* [WIP] started on source deletion

* Removed unneeded test blocks

* Added marked_for_deletion_at to sources and media profiles

* Hooked up async deletion to media profiles as well

priv/repo/migrations/20240722183656_add_marked_for_deletion_at_to_sources_and_profiles.exs

@@ -0,0 +1,13 @@
+defmodule Pinchflat.Repo.Migrations.AddMarkedForDeletionAtToSources do
+  use Ecto.Migration
+
+  def change do
+    alter table(:sources) do
+      add :marked_for_deletion_at, :utc_datetime
+    end
+
+    alter table(:media_profiles) do
+      add :marked_for_deletion_at, :utc_datetime
+    end
+  end
+end