V60: Email Templates UUID FK Columns

Adds created_by_user_uuid and updated_by_user_uuid columns to phoenix_kit_email_templates. These columns are referenced by the PhoenixKit.Modules.Emails.Template schema but were never created by V15 (which only had integer FK columns).

Fresh installs now get these columns from V15 directly. This migration covers existing installs that already ran V15 without them.

Idempotency

Uses column_exists checks — safe to re-run and safe on fresh installs.