Reactivate Subscription

Reactivate a subscription that was set to cancel at period end.

Only works if the subscription hasn't actually been deleted yet (status is cancel_pending, Stripe still has it as active with cancel_at_period_end=True).

Phase 3 (2026-04-26 fix): brand_id 対応。multi-brand owner で brand_id を 渡さずに呼ぶと 400 で拒否。1-brand owner は brand_id 省略可 (legacy 互換)。 DB UPDATE は WHERE id = (target row id) で 1 行限定 — owner_id 単条件で 全 sub の status を上書きする旧バグの再発を防ぐ。