Migration: goal custom props + revamped unique constraints (#5940)

* Migration: add custom propos to goals + revisit unique constraints

* Fixup migration

priv/repo/migrations/20251209120138_goals_custom_props.exs

@@ -0,0 +1,41 @@
+defmodule Plausible.Repo.Migrations.GoalsCustomProps do
+  use Ecto.Migration
+
+  @disable_ddl_transaction true
+  @disable_migration_lock true
+
+  # adds custom_props to goals, but also updates unique constraints to guard for either display name 
+  # or broadly speaking "goal configuration"
+
+  def change do
+    drop(unique_index(:goals, [:site_id, :event_name], name: :goals_event_name_unique))
+
+    drop(
+      unique_index(:goals, [:site_id, :page_path, :scroll_threshold],
+        name: :goals_page_path_and_scroll_threshold_unique
+      )
+    )
+
+    drop(unique_index(:goals, [:site_id, :display_name], name: :goals_site_id_display_name_index))
+
+    alter table(:goals) do
+      add(:custom_props, :map)
+    end
+
+    create(
+      unique_index(:goals, [:site_id, :event_name, :custom_props],
+        where: "event_name IS NOT NULL",
+        name: :goals_event_config_unique
+      )
+    )
+
+    create(
+      unique_index(:goals, [:site_id, :page_path, :scroll_threshold],
+        where: "page_path IS NOT NULL",
+        name: :goals_pageview_config_unique
+      )
+    )
+
+    create(unique_index(:goals, [:site_id, :display_name], name: :goals_display_name_unique))
+  end
+end