V65: Rename SubscriptionPlan → SubscriptionType

Renames the subscription plans table and related FK columns in subscriptions to reflect the correct "subscription type" naming convention.

Changes

1. phoenix_kit_subscription_plans table → phoenix_kit_subscription_types
2. phoenix_kit_subscription_plans_slug_uidx index → phoenix_kit_subscription_types_slug_uidx
3. phoenix_kit_subscriptions.plan_id → subscription_type_id
4. phoenix_kit_subscriptions.plan_uuid → subscription_type_uuid

All operations are idempotent — safe to run on any installation.