Page Scroll Goals: migration step 1 (#5033)

* migration: add scroll_threshold to goals

* do not drop the old index yet

Author: RobertJoonas

priv/repo/migrations/20250128161815_add_scroll_threshold_to_goals.exs

@@ -0,0 +1,29 @@
+defmodule Plausible.Repo.Migrations.AddScrollThresholdToGoals do
+  use Ecto.Migration
+
+  @disable_ddl_transaction true
+  @disable_migration_lock true
+
+  @new_index unique_index(
+               :goals,
+               [:site_id, :page_path, :scroll_threshold],
+               where: "page_path IS NOT NULL",
+               name: :goals_page_path_and_scroll_threshold_unique
+             )
+
+  def up do
+    alter table(:goals) do
+      add :scroll_threshold, :smallint, null: false, default: -1
+    end
+
+    create(@new_index)
+  end
+
+  def down do
+    drop(@new_index)
+
+    alter table(:goals) do
+      remove :scroll_threshold
+    end
+  end
+end