V116: Parent reference on entity_data.

Adds a nullable self-referential parent_uuid column on phoenix_kit_entity_data, letting each data row point at another row of the same entity as its parent. The feature is a system field on every entity_data row — it is always present, optional to fill, and never removable by the user (it does not appear in entities.fields_definition). Existing rows stay parent_uuid = NULL and become roots; no backfill needed.

The column is nullable (roots have no parent) with no ON DELETE cascade: parent/child linkage and same-entity scope are managed by the PhoenixKitEntities.EntityData context, which runs subtree checks inside a transaction. A DB-level cascade would bypass the soft-delete machinery and the activity log.

Same-entity enforcement (a row's parent must share its entity_uuid) is a context-layer responsibility — the self-FK has no view of entity_uuid, so the changeset + context perform the lookup before saving.

A plain b-tree index on (parent_uuid) covers the "list children" query used when rendering the WordPress-style indented tree. Existing indexes on (entity_uuid) and (entity_uuid, position) still cover per-entity listing and manual ordering.