ReactivatePaidSubscription

Validates POST /v1/billing/reactivate (D-32 + D-42). Body: empty (the action is unconditional — caller asks to undo a pending cancel-at-period-end). Headers: Idempotency-Key: required UUIDv7 (D-32). Forwarded to Mollie so retries collapse on the provider side. Authorization: Owner-only (D-42). Mirrors the inline-pivot pattern from {@see PlanChangePreviewRequest}.