Saved segments/create table (#4797)

* Add migration for Saved Segments

* Remove premature optimisation

* Format

* Refactor to explicit segment type

priv/repo/migrations/20241107120000_create_segments.exs

@@ -0,0 +1,22 @@
+defmodule Plausible.Repo.Migrations.CreateSegments do
+  use Ecto.Migration
+
+  def change do
+    execute(
+      "CREATE TYPE segment_type AS ENUM ('personal', 'site')",
+      "DROP TYPE segment_type"
+    )
+
+    create table(:segments) do
+      add :name, :string, null: false
+      add :type, :segment_type, null: false, default: "personal"
+      add :segment_data, :map, null: false
+      add :site_id, references(:sites, on_delete: :delete_all), null: false
+      add :owner_id, references(:users, on_delete: :nothing), null: true
+
+      timestamps()
+    end
+
+    create index(:segments, [:site_id])
+  end
+end