PlanChangePreview

Validates POST /v1/billing/plan-change/preview (D-40 + D-42). Body: from_tier_id: required UUID — current paid tier the household is on. to_tier_id: required UUID — target tier the user is considering. Authorization: Owner-only (D-42). The pivot row on `account_household` is looked up by (user_id, household_id) and rejected unless its `role` casts to {@see AccountHouseholdRole::Owner}. Laravel surfaces a `false` return as 403 Forbidden, no literal status code is touched in user-code. Per CLAUDE.md "DI over facades" — constructor-injects the {@see CurrentHouseholdResolver} contract so the authorize() pivot lookup can resolve the caller's active household without touching the auth/config facades. Laravel resolves FormRequest constructors out of the container.