V63: UUID Companion Column Safety Net — Round 2

Adds missing uuid and _uuid FK companion columns identified after V62. All operations are idempotent — safe to run on any installation.

Issues Fixed

1. phoenix_kit_ai_accounts missing uuid column

V61 used the wrong table name in @tables_missing_uuid (listed phoenix_kit_ai_requests instead of phoenix_kit_ai_accounts). This left ai_accounts as the only legacy table without a uuid identity column.

2. phoenix_kit_ai_requests missing account_uuid companion

The account_id integer FK had no UUID companion. Backfilled via JOIN to phoenix_kit_ai_accounts once that table has its uuid column.

3. phoenix_kit_email_orphaned_events missing matched_email_log_uuid

The matched_email_log_id integer FK had no UUID companion. Backfilled via JOIN to phoenix_kit_email_logs.

4. phoenix_kit_invoices missing subscription_uuid companion

The subscription_id integer FK had no UUID companion. Backfilled via JOIN to phoenix_kit_subscriptions.

5. phoenix_kit_shop_cart_items missing variant_uuid companion

The variant_id integer FK had no UUID companion. No backfill possible (no variants table in schema); column added as nullable for future use.